Never odd or even
Ten years of off and on experimentation and research. That's what it had taken to bring Twilight to the precipice of magic once more. Of course, ruling Equestria took up much of her time. Enough that she could have been done within a year had she no other responsibilities.
She had tested her work before, failing multiple times, but she was confident that this was it. Probably.
Closing her eyes, Twilight began casting her carefully crafted spell. Alone in her office, there was no buildup or fanfare as half of the spell completed without a hitch. The other half would be completed by the mirror of herself in another universe. Together, the two spells connected and surged together into a unified whole.
Twilight opened her eyes and saw herself. A grin formed on the mirror world Twilight's face at the same time as it formed on her own. Floating above the floor with barely enough space for a coffee mug, the portal glowed purple around the intersection of worlds.
Both of the Twilight glanced at the clock on the wall and immediately resigned themselves to staying up all night studying the effects of the portal. They had a lot of work to do.
Twilight concluded, in the most horrifically frustrating way possible, that the universe on the other side was a perfect mirror of her own. Twilight looked at her mirror self, dark circles under her eyes, mane unkempt, and discouraged as can be.
As quickly as possible, in an attempt to catch her mirror by surprise, Twilight levitated her empty coffee mug off of the ground and chunked it at the portal at speeds reaching near mach 1. Upon reaching the portal, it met its equal at the threshold, the two colliding in perfect unity of motion without a mote of dust crossing the practically mundane horizon.
It was futile. Nothing could get through without its counterpart meeting it in the middle. Thirty years of research for a fancy mirror that, instead of showing a reflection, it showed a pony so alike to yourself that you were practically the same creature. Everything was mirrored perfectly. The spell was too perfect and the universes too alike.
Thirty years wasted on a fancy mirror. Not even Pinkie could outplay herself and cause some break in the symmetry. It was basically useless.
Twilight looked at her mirror self and had a thought that she hadn't dared entertain before. She could kiss another pony.
Twilight Sparkle, Princess of Equestria, could have her first kiss with…another alicorn who just happened to look and behave exactly like herself. She felt her cheeks burn at the thought, glancing around her empty office in paranoia to make sure nopony was watching. A glance at the mirror world told her that somepony else was watching. Twilight Sparkle was watching.
With shaky legs, Twilight stood and looked into her eyes. The other Twilight was blushing.
With slow and hesitant steps, Twilight made her way to the mirror and stopped with her face a hoof's width away. She couldn't feel the other Twilight's breath due to the air colliding halfway in between them, but she could hear the tiniest of delays between sounds made by herself and ones made by the mirror Twilight.
No words were said, as both were certain of the intent of the other. They both leaned forward slightly, and their lips touched in the center. Twilight kept her eyes on her mirror self, watching her reaction to determine her own. Her eyes were dilated slightly, and the blush had not left the mare's cheeks. Twilight decided that she rather enjoyed kissing.
She closed her eyes and opened her mouth a bit, poking her tongue out to meet its twin halfway. She toyed around with her tongue for a few minutes, becoming lost in the intimacy of the moment, before tilting her head to get a better angle in the kiss. Her double tilted her head as well…in the opposite direction.
Twilight's eyes shot open. Staring directly into the mirror world, Twilight saw her double's neck and shoulder. Twilight took a step back, her double doing the same.
"We'll call this direction 'tilt forward,' it is to the…"
Twilight looked her mirror self in the eye, both of their heads still tilted.
"[Left/Right]" They said simultaneously.
The Twilights grinned.
Knowing what her other self was likely thinking, she turned her body to face her clock on the right wall. Her double turned as well, facing her mirror clock on the mirror wall. Now, their heads were tilted in the same direction, relative to each other.
Time for a test.
"The Twilight who's horn is facing [away from/towards] the portal steps through."
The Twilights chuckled.
"You just want to be the first to step through." They accused each other.
Twilight stopped bothering to talk to herself, assuming that they were still synchronized in every way except for their head tilt direction. With a thought, she levitated a glob of ink from her inkwell and formed it into an arrow facing the direction her head was tilted. She stamped the shape of the arrow onto the ground in front of the mirror.
Looking up, Twilight saw a distinctly not mirror version of her arrow on the opposite side of the portal. They both shuffled slightly in the direction of their respective arrows, right for Twilight and left for mirror Twilight.
Twilight snickered, her mirror giving her a confused look.
"I'm Twi-right." Twilight explained.
"I didn't think of that." Her mirror said with a snicker of her own.
They both gasped.
"We're having a conversation! The difference in direction altered our perception of ourselves relatively speaking! We're not perfect mirrors anymore!" The two shouted in sync.
They both tilted their heads in the direction they had when they kissed. "In the direction of tilt? Tiltward? Tiltwise? Tiltwise sounds good. Now we need to figure out how to desynchronize our speech at will. Given how quickly we returned to being synchronized, we should create a way of removing synchronization based on something that will give consistent results when synchronized."
The Twilights thought for a moment.
"If we use a dice, we can assign one of us to do something first based on if the dice lands on an even or odd number. I've always felt like right was more even and left was more odd."
Without another word, the two Twilights teleported from their respective offices.
Twilight appeared in the barracks in a flash, looking around at all of the surprised faces of the off duty guards staring at her. In a clamor of commotion, they all scrambled into something resembling order, standing at attention.
"Does anypony have a die that I can borrow?" She asked.
Several of the guards darted off to the ends of their bunks and retrieved dice. Twilight picked one at random.
"Thank you!" She said before teleporting back to her office.
Facing the portal, Twilight and her mirror rolled the dice, both getting a three.
"[YES!/NO!]"
"Wait! Same time! We should go at the same time!" Twilight said in a panic. She did NOT want to be stuck being the second pony through the portal. She was half responsible for it!
"Oh, I didn't think of that."
"We didn't." Twilight corrected.
Mirror Twilight rolled her eyes.
"Well, now we're not synchronized again." Mirror Twilight said. "We need to figure out how to return to synchronization."
"Teleporting away seemed to work just fine." The two said together. "Or it just happens on its own like that. So not only is your world a mirror of mine, but it attempts to remain a mirror of mine after changes have been made. That has disturbing implications for free will."
Deciding to ponder her questions later, Twilight took a step tiltwards and her mirror did the same. The two kept their eyes on each other as they stepped forwards, nearly kissing again as they passed through the portal due to the close distance. Both stopped halfway through, looking down at the other's flank and watching the other's tail slowly rise.
Twilight looked her mirror in the eye again.
"We should talk to Cadence about this." They said.
Passing through the portal completely, Twilight looked around the strange mirror version of her office.
After looking around for a bit, Twilight walked back over to the portal, facing her mirror self. The fact that the other mare staring at her so intently made her uncomfortable. They were, for all intents and purposes, the same mare. It would be nice to have somepony that she could work with and trust on that level, but being so similar could potentially create an identity crisis later on.
"We'll need some kind of naming difference between the two of us." Twilight and her mirror self said.
"I'm okay with Twi-right." Twilight said.
"Twi-left just sounds weird though." Her mirror self answered. "How about just Twilight, and we make a point of having our hair and any accessories asymmetrical?"
"No, we need to be uniquely addressable by third parties. How about Twi-Right and Tilt-Twi?"
Her mirror shook her head. "Twi-Right sounds fine, but nothing for me seems normal."
Twilight cringed a bit, but spoke her thoughts anyway. "You could just keep being Twilight and I'll change?"
"I don't want to do that to you. It would be pretty unfair."
"Maybe we could keep it until we think of something else?" Twilight offered.
"No, maybe we just say that we are the native or foreign Twilight based on where we are?"
"Maybe not specifically the words native and foreign though," the Twilights said together, "I don't want to be foreign to my friends on this side. Synchronized again."
The Twilights picked up their respective dice and rolled them, both coming up with a two.
"Maybe we could be referred to as 'our Twilight' and 'mirror Twilight' or some variation thereof by our friends?" Twilight asked.
"That's what I was thinking." Mirror Twilight said, sticking out her tongue.
They smiled a bit at each other. Twilight decided that it was mirror Twilight's turn to start, so she remained silent, waiting for her to say something.
"What are we going to do about wanting to…have sex."
That actually caught Twilight a bit by surprise, which was itself, surprising. The mirror Twilight seemed to be surprised that Twilight was surprised. It was all very surprising.
Twilight pulled out a quill and paper from her mirrored desk as her mirror self pulled out the same from her regular desk. A list was sorely needed.
Option 1: Have sex.
Option 2: Do not have sex.
Option 3: Talk about the problem with friends/family before deciding between the above.
Option 4: Close the portal so that it is no longer a concern.
The Twilights glanced up at each other, making sure that they were synchronized.
"Odds take friends." They said before rolling their dice. One on both. Oops.
Twilight watched passively as the mirror Twilight rolled again. Five. Mirror Twilight would list friends.
Twilight turned back to her list.
Shining Armor
Pros: BFF, unlimited trust, able to keep private things private.
Cons: BB, awkward to discuss sexual things with, likely to talk with Cadence about it even if we don't.
Cadence
Pros: Princess of Love, unlimited trust, able to keep private things private.
Cons: Easily excitable (not sexually) on matters of love and sexuality, likely to talk with Shining Armor about it even if we don't.
Parents
No.
Celestia
Pros: High level of experience, wise, unlikely to outwardly react strongly or primarily from emotion.
Cons: More awkward than talking to Shining Armor about it, might have a similar interest in self given her age, unlikely to give the most helpful advice to allow us to figure things out on our own.
Luna
Pros: High level of experience, wise, unlikely to consider it particularly strange due to having to rapidly adapt to modern sexual standards and cultural changes.
Cons: Going to her for advice is uncommon, thus making things awkward in a different way. Might have a similar interest in self given her age.
Twilight scrunched her nose at having changed her list format for Luna's 'Cons' entry, but could not immediately think of an easy way to remedy the issue.
"What have you got? I've got Rarity then Fluttershy." Mirror Twilight said.
"Shining Armor and Cadence. No surprise there." Twilight responded.
"Now which ones do we go see?" Twilight asked.
Twilight thought for a moment.
"Why did-" They said simultaneously, pausing to roll the dice. Three.
"Why did you pick Shiny and Cadence?" Mirror Twilight asked.
"They were the best options. Rarity and Fluttershy?"
"They were the least terrible options." Mirror Twilight snickered.
"Alright, we'll talk to our respective parties, then meet back up once we've gotten some advice."
The Twilights nodded before turning to return to paperwork...in the wrong direction. With a simultaneous blush, they realized that they were still on each other's side of the portal.
Twilight walked up to the mirror, die in her magic, and rolled. One.
"Rarity and Fluttershy gave an overall positive response." Mirror Twilight said. "They had concerns, but I want to know what Shiny and Cadence had to say first."
"Shiny and Cadence gave an overall positive response." Twilight answered. "They expressed concerns about me, or I guess us, socially folding in on ourselves and developing an unhealthy dependency on one another. I am 85 percent for doing this."
"Fluttershy was too embarrassed to say much," mirror Twilight said, "but Rarity expressed concerns about how such a relationship might be interpreted by the public if this was to get out. I am 95 percent for doing this."
Both Twilights took a deep breath and nodded in sync. They took one step tiltwards, Twilight stepping to her right and mirror Twilight to mirror left, and walked forwards. Stepping through the mirror, Twilight ogled her mirror's flank and lifted tail and the knowledge that her mirror was doing the same made her tail lift all the more.
Slowly, Twilight stepped forwards enough for her mirror's vagina to come into view. Blush on her cheeks and a tingle in her nethers, Twilight tentatively sniffed at it.
With butterflies in her belly, Twilight stuck out her tongue and ran it up mirror Twilight's labia, shuddering as she felt the same done to her. Bringing her muzzle down a bit, Twilight prodded her clitoris with her tongue, feeling the shape of it. Curious and excited, Twilight inserted her tongue into the mare's vagina and quickly located the vulvo-vagina sphincter and urethra, briefly licking at both before moving on to attempt going deeper.
Thankful for the length of her tongue, Twilight reached the cervix and began exploring around the fornix before gently flicking the dorsal frenulum. Twilight gasped at the intensity of the feeling as her mirror self did the same. Nervously, Twilight pushed her tongue against the external uterine ostium in an attempt to enter her cervical canal, but the reach of her tongue and the size of the opening worked against her. Even still, the resulting pressure on her cervix and compression of her vesicogenital peritoneal pouch was intensely pleasurable.
Gently, Twilight relaxed and extended her tongue, applying and releasing pressure on her cervix. The roughness of her mirror's tongue, the pressure, and the mental awareness of having oral sexual intercourse with somepony who was external to herself quickly brought Twilight to have an orgasm.
Her paraurethral glands and bladder were quickly overwhelmed, secreting ultrafiltrate blood plasma and urine in a torrent of ejaculant. Mouth wide open and tongue deeply inserted in the vagina, Twilight moaned as the liquids mixed and poured around and over her tongue. Her tongue and mouth, acting as a moderately ineffective stopper at the entrance of the vagina, foiled the biological safeguards against intravaginal pooling and caused some of the ejaculant to flow deeper into the vagina.
Eager to prevent a more concerning development, Twilight desperately tried to plug her mirror's cervical canal with her tongue as she rode out her orgasm. In her orgasmic panic to prevent potential health problems, Twilight failed to properly close her esophagus and it was quickly inundated with her mirror's ejaculant.
With a sputtering, wet cough, Twilight felt the liquid surge into her sinuses and out through her nose. The pleasurable and painful series of events cause her forelegs to collapse and her tongue to withdraw. In a moment, she was coughing on the floor in a pool of yellow ejaculant that consisted primarily of urine.
As her coughing subsided, Twilight picked up her mirror's die beside her and rolled it, idly noting that the three was slanted in the wrong direction relative to the locations of the other numbers. Six.
"Unnngggggg that hurt so good." Twilight muttered just loud enough for her mirror self to hear.