"Well," Sunset said as she looked between the two Pinkies, "At least I don't have to worry about telling you apart. Did either of you want a drink?"
"Just my usual," both Pinkies said at the same time, then broke into giggles.
Once Sunset left the living room, the native Pinkie Pie turned to her cross-dimensional self. "You don't have to go along with any of this. We kinda got together sudden-like, and we're still feeling out all the things that we enjoy together."
"Pfft! Please, you'll barely notice me. It's kinda odd, since there isn't a Sunset in Equestria, well, there's a sunset, but just not a Sunset. Princess Celestia makes sure that—" Pony Pinkie froze at the site of a finger on her lips, then giggled.
"Now I know what that's like… Interesting. Anyway, it's up to you what you want to do. But, there's one thing you can't do." As she said it, Human Pinkie felt a chill that told her she was doing something wrong. "Err, that is, it's dangerous to do. Marble has these worm things in the basement, don't go near them and don't let them out. They like to get into your head a little too literally, and they make you want to do things."
For a second Pony Pinkie had every intention of finding out what her cross-dimensional twin sister was playing with, but then she heard the full extent and instead turned her floofy-haired head to the side a little. "Do things?"
"Well, since Rainbow got one in her head, she can't stop wanting to have sex. All the time. I mean, it seems like something fun, but all the time is just a little too much, right?"
"Yeah. You have to have the right balances of fun, dontcha? Sexy-fun is not the kinda fun you want at a birthday party, unless it's a special birthday party where you wear birthday suits." Human Pinkie waggled her eyes, and was happy to see Pony Pinkie giggle appropriately. "Are, um, you actually up for this?"
Pony Pinkie nodded. "We're all girls, so there's no trouble of whoopsie-foal-batter-in-your-oven-ie, and if something goes horribly wrong, I'll skedaddle back to Equestria and give you two some time to talk about it."
"No, that wouldn't be fair. If this goes wrong, we all should talk about it." Human Pinkie just finished talking as Sunset walked back into the living room with a tray of drinks and a glass jug of water. "We were just talking about having some fun."
Sunset's wings fwoomped out and she almost dropped the tray. "Wait! Hold it! What kind of fun? The kind where you plan a big party for everyone and keep swapping all night so they never know which Pinkie is which or—"
"Actually, the kind where we both kiss you at the same time," Human Pinkie said.
"We could do more than kiss, right?" Pony Pinkie asked. "Only, when I've been with mares back home we did a lot more than just kiss. Also, I'm pretty sure we have similar enough bits that we can do more than kiss."
With that, Pony Pinkie reached to the waistband of the stylish shorts and pulled it forward so she could look down. "Yup! Definitely similar enough. Though it looks a bit different. More foldy bits. Wanna see?"
"Later," Human Pinkie said.
"No," Sunset said.
Human Pinkie giggled. "Later if you want to."
Sunset froze, staring at her girlfriend. "You mean you're okay with this? Really okay? Like, not just wanting to because I really want to and you're actually—" Sunset's rambling question was both answered and silenced by Human Pinkie kissing her. She wanted nothing more than to melt into Pinkie's embrace, but the tray was still in her hands.
Pony Pinkie took the tray from Sunset's hands and set it on the coffee table. With the couple kissing, she felt a little left out and looked around the room for something to amuse her. When her gaze dipped down, Pinkie paused and looked at her chest.
Having spent plenty of days in the human world before, Human Pinkie knew what breasts were, but having never really pondered intimate things before, she'd never really explored them beyond noting they felt nice to squeeze.
Experimentally, Pony Pinkie slipped one hand up her own shirt until her palm cupped the fabric of something that hugged her chest. Knowing what a bra was, she teased the front of it down and felt her chest first compress, then spring forward as she escaped the restrictive device.
"What are you doing?" Sunset asked when she broke from the kiss and saw Pony Pinkie playing with her breasts—her rather full breasts.
"Just playin', I guess. Never really gotten into the mood for snuggles and fun when I've been here."
"Well," Human Pinkie said, "Why don't we go to our bedroom and play together?"
Sunset Shimmer hadn't enjoyed a lot of relationships in her life, and none apart from Pinkie Pie had been in any way equal. Pinkie made her feel both appreciated and loved, and above all she knew Sunset inside and out. "Are you sure you're both okay with this?"
Pulling her hand out from her top, Pony Pinkie gave Sunset her biggest smile—which was pretty big. "Like I said, we can't get actually hurt by this, and if we're all having fun…"
"Fun!" Human Pinkie said. "Come on, Sunny, you'll have fifty-percent more boobs to play w—" She froze in place, her eyes widening in shock. "I-I'll have fifty-percent more boobs to play with."
Human Pinkie's state was enough to finally break Sunset's resolve. Sunset could see for herself that her girlfriend was into the situation for more than it just being something Sunset wanted. "Well, let's go then."
As they filed down the hallway, Twilight Sparkle opened her bedroom door and stopped. "Oh, Hi Sunset!"
"Hi Twilight," Sunset said and walked on to her bedroom.
"Hi Pinkie!" Twilight blushed a little at seeing the couple together on the way to their bedroom. Her mind ran wild with possibilities.
"Hiya Twilight!"
"Hi Pink—" Realization sank in as the second Pinkie Pie passed Twilight's door, that something odd was going on, but Twilight was struck dumb by the realization that there really was a second Pinkie.
By the time Twilight got her faculties in order and poked her head into the hallway, all three girls were in the bedroom together and Sunset was just reaching out to put a coat hanger on the door. She opened her mouth to say something, but it was too late—Sunset had closed the door.
"Oh. My. Goodness! Did you see her expression?!" Sunset turned around to face the bed, only to be cut short in her celebration of the gag by the sight of two Pinkies with their tops off. While it wasn't hard to tell them apart by their state of being ponied up, Sunset couldn't help but compare one aspect of them. "Are our boobs shrinking when we pony up?"
"Hrmm…" Human Pinkie turned to her counterpart, focused on Pony Pinkie's boobs, and then giggled. The change was immediate and startling. Fur sprouted all over her, her mane grew into an even larger poof of frizz, and her face pushed out into a muzzle. When she looked down, then looked back at Pony Pinkie's chest, her hair seemed to lose its bounce. "Y-Yes."
Walking across to her girlfriend, Sunset did the best thing she knew to cheer Pinkie back up. Pulling off her top, Sunset unclipped her bra, took hold of Human Pinkie's cheeks with her palms, and guided Pinkie's face between her breasts. "You've still got these."
"Iiiii'm picking up a bit of a vibe here. Does somepony really like boobies?" Pony Pinkie lifted her arms up and shook her chest. When both Sunset and Human Pinkie stared at her display, Pony Pinkie knew she'd guessed right. "Why don't you lay back, little ponk, and let your special friends show you what you have to live for?"
Sunset picked up on Pony Pinkie's plan right away. "Go on. Lay down on the bed—on your back." She guided Human Pinkie down until she was laying as Sunset told her. "Now, I think one of us on each side should be enough to perk up my pony girl."
Looking up from where she lay, Human Pinkie's eyes were wide as Sunset lay on her left and Pony Pinkie on her right. Sunset's breasts pressed against her cheek, and Pinkie couldn't help a giggle of utter delight.
When a pink pair of breasts came in on her other cheek, Human Pinkie let loose a pure laugh and reached up a hand to play with one each.
"These are fun to play with." Pony Pinkie had her chest pushed forward to help cheer up her human self, and while one of Human Pinkie's hands massaged her breast, her own hand took care of its twin. Though she could see another target.
Cupping Human Pinkie's smaller breast with her free hand, Pony Pinkie quickly found ways to encourage excited little gasps from the girl with gentle massage and a playful thumb stroking her nipple. "What's it like back—err, down, I guess? Completely different, or is it much like a pony once you get the hang of it?"
Sunset leaned over Human Pinkie a little, half smothering the girl with her breasts. She was happily surprised when Pony Pinkie leaned in too and they shared a little kiss. There was a thrill to it. Kissing another girl while spread over the top of her girlfriend was exciting, but not as exciting as what Sunset wanted to say. "Why don't you head south and take a look?" she asked when the brief kiss broke.
Under a double helping of breasts, Human Pinkie let out a happy—muffled—moan of arousal. The depression of her own chest shrinking had been more than balanced by an extra serve of her favorite body parts, and when she heard Sunset's invitation, she rubbed her thighs together in excitement.
"This one looks just like a pony's!" Pony Pinkie said.
"Try the other!" Human Pinkie grinned up at Sunset, but her distraction made her all the more surprised when a finger ran up the middle of her vulva and put pressure on the thick lips of her marehood. "Oh golly!"
"Golly?" Sunset asked. "Pinkie, we need to get you more—Ohhh…"
Using one hand to tease at Human Pinkie's equine vulva (something Pony Pinkie knew how to play with already), Pony Pinkie focused more attention between Sunset's thighs. The delicate folds of a female human's body were different than what she was used to playing with, and it led her to explore with her mouth while her one free hand supported her.
Parting her legs wider, Sunset slid her belly down onto her girlfriend's midsection and pressed her chest down against Pinkie's as well. Biting her lip against the exploring tongue, she tried to say something but had to give up and just moan.
"She's really—oh heck—really doing a number on—nnnnnggggg—on youuuu!" Human Pinkie finally howled out and squirmed as one of Pony Pinkie's fingers pressed into her. Rocking her hips, she kicked her legs as the finger stroked in and out of her pony vulva.
Between nuzzles and licks, Pony Pinkie managed to say, "You two are—really getting—into this—huh?" She almost seemed heedless of how much she played the pair. Almost. If there was one thing Pinkamena Dianne Pie enjoyed more than anything else, it was making ponies (and now people) happy, and by the sound of her trans-dimensional self and her trans-dimensional self's girlfriend, she was doing a great job of it.
But Pony Pinkie wanted more. She transferred her mouth from Sunset's human vulva to Human Pinkie's pony one, which gave her a better look at Sunset's body. She looked at the delicate, and now engorged, flesh and was able to bring both hands up to explore. With a stroke, she found several places that made Sunset squirm, and set about making her squirm more with one hand while exploring with the other.
Human Pinkie's eyes crossed as Pony Pinkie lapped at her vulva. She squirmed and wiggled, then broke into giggles. Only once she'd laughed her way through an orgasm did she decide it was time to change things up. "Alright, that's—" Human Pinkie giggled a little more at some more nuzzling licks, "—that's enough. Time to return the favor."
When Human Pinkie grabbed Pony Pinkie's legs and heaved her around, she wasn't sure what was about to happen, then Human Pinkie pressed her head between Pony Pinkie's thighs and let the girl know exactly what the plan was.
Seeing her girlfriend and her girlfriend's doppelganger getting more intimate, Sunset barely had to think before she reached a hand out to each and—touching one on the thigh and the other on the shoulder, tried to pull them both into the same mindscape.
Human Pinkie blinked in surprise and looked at Sunset. "You only got one of us."
A moment later and Pony Pinkie was in Sunset's head instead, only now there was no Human Pinkie. "Ooooh! What's this? Is this inside your head? It's nice in here."
"H-Hold on a second. I can get this—" Sunset leaned harder on her talent and pulled with all her might, "—working!" She looked at the pony and the human girl standing before her. "There."
"Sooo…" Human Pinkie looked around. "This is different."
"How do you mean?" Pony Pinkie asked.
"Well, normally I pull her into my head. It's not as well organized as this. It's more of a nightclub than a library."
Pony Pinkie lifted her hoof up and tapped the side of her head in thought. "But this is just a dream-thing, right?"
"Weeeeeellll, it's more like the inside of Sunset's head. This is her, so the first thing we need to do is protect all this. A drop-cloth and some plastic—" As Human Pinkie spoke, she gestured around as coverings fell all over the orderly part of Sunset's mind. "There. That should keep all that safe."
Sunset watched in surprise as the cozy part of her mind was sealed over and protected. A slight thickness came to her thoughts, but with some effort she pushed through it. "So I was thinking a hot-tub and—"
Human Pinkie, now wearing a tight swimsuit over her heavy assets, sauntered toward Sunset. "A hot-tub is a good start, Sunny."