A zipper opened in the air above a bit of ground with a swirl of colors as its sky. It unzipped with an appropriate sound and a purple alicorn (or was it lavender?) stepped through... upside down. She fell onto the platform very ungracefully.
"Discord!" Twilight P said.
"Oh, watch that first step," Discord said, his head poking through the zipper upside-down.
"Now he remembers," she said.
Discord withdrew his head and the two unicorns passed through, these carefully stepping over the zipper-threshold to deal with the inverting of gravity. The dragon attempted to fly in, but still managed to collapse unelegantly onto the platform. Discord floated in upside down last, zipping the portal up behind him. This done, his head and appendages detached and floated around to orient him right-side up.
"Is Celestia not coming?" Twilight P asked.
"Oh she said something about the safety of the kingdom and blah blah blah," Discord said, waving his claw in the air dismissively. He then disappeared and reappeared right behind Twilight P, his arm over her haunch (shoulder) and stage-whispered in her ear.
"Between you and me, I think she was waiting until we were gone to start laughing."
"Celestia would never do that!" Twilight P said.
"Eeehhh..." Spike said, wobbling his hand to indicate uncertainty.
Sunset was supressing a chuckle.
"Point taken," Twilight P said, "So, Discord, what's this about me 'desecrating' your home?" The air quotes weren't gestured but they were audible.
"Take a look," Discord said, gesturing out into the chaotic space.
A few dozen meters (or yards if you prefer) away was a translucent Twilight Sparkle tumbling through space. In the distance, more Twilight Sparkles. Some tumbled without moving themselves, some were flying without any apparent purpose, and a few were flying or walking around and talking to various trees and rocks.
Discord sprouted two additional arms and closed all four visitor's mouths at once.
"What but... how? Why?!" Twilight P exclaimed.
"I'd hypothesize it had something to do with your newfound ability," Twilight H said, shoving her glasss back into position.
"Yeah, Twilight, that was pretty obvious," Spike said, folding his arms.
"I mean, yes I figured that," Twilight P said, "But still..."
"Oooh, scientific exploration!" Discord said, a labcoat and goggles appearing on him. He now held a beaker of bubbling green liquid in one claw. It stopped bubbling and he looked at it with some confusion, putting his eye up to the top. A plume of fire them emerged, leaving Discord's face scorched. Discord looked over at the visitors expectantly. The Twilights stared at him skeptically in stereo. Spike raised an eyebrow. Sunset, after a brief pause, chuckled politely.
"Thank you!" Discord said. He then poofed back to normal. "Honestly, the Sparkle family has no sense of humor."
"It wasn't really your A material," Spike said by way of constructive criticism, "Maybe if you had turned into one of those balls with the other balls around them?"
"An atom?" Twilight H asked, "You know about those?"
"Oh yes!" Twilight P said, suddenly excited, "You see, magic can be used to probe small scales in ways comparable to..."
"Ahem, can we get back to my problem?" Discord said. He was now wearing a brown jacked, gloves, a scarf, different goggles, and an odd hat that went down the sides of his head, where ears would be in a human. The humans recognized it as an old-timey driving outfit. He was also in a sitting position, though there was no chair visible.
Discord mimed adjusting a gear shifter and stepping on a pedal and the rock they were on began moving toward the nearest ethereal Twilight with a sputtering sound.
"You've not even seen the worst part yet," he said as he pulled the rock up next to the Twilight image.
"It is strange," Sunset said, "And unsettling for Twilight, sure, but what...?"
"Hi, I'm Twilight Sparkle!" the shimmering Twilight said, "I like books and friendship!"
"I do not sound like that!" Twilight P said indignantly.
"Eh," said Spike, waving his claw in a 'kind of 'gesture.
Sunset and Twilight H suppressed giggles. Discord laughed a bit, then suddenly turned serious.
"Yes yes, most amusing," he said, "The first dozen times. After that it's just... predictable."
"Hi, I'm Twilight Sparkle!" the shimmering Twilight said, "I like books and friendship!"
"You see?" Discord said, "And it gets worse." He snapped his claws and they reappeared in his house. Fluttershy was flying up near the ceiling, trying to corral one of the transluscent Twilights.
"I like books and friendship!" it said.
"Yes, I know," Fluttershy said, with only a hint of annoyance in her voice, "But please come down into the kitchen? Um, if you don't mind."
"Do I even want to ask?" Twilight P said. Discord walked over and cracked open the door to the next room. A mix of voices came out and a number of hooves and wings pressed at the opening. He put his shoulder to it and shoved the door closed again.
"Oh, Twilight, you're here!" Fluttershy said, "And... you're also here?" She took a moment to assess the glasses-wearing Twilight.
"Hi, I'm Twilight Sparkle," Twilight H began, extending her hoof. Then she realized how that sounded. "Uh, I mean, I'm the Twilight from the other side of the mirror. And I'm guessing you're this world's Fluttershy. Do you like animals as much as our Fluttershy does?" *
"Oh, I love them!" Fluttershy said, momentarily forgetting the transluscent Twilight, who continued to fly around the ceiling in lazy circles. "Does my counterpart have an Angel Bunny?"
"She sure does!" Twilight H said, "She volunteers down at the animal shelter almost every day after school, too."
"Oh, well I recently opened my own animal sanctuary," Fluttershy said, "I hope my counterpart can do as well. I'm sure there are lots of cute little animals over there who need a safe place to stay."
Discord cleared his throat. "Ladies? The issue at claw?"
Spike, while this conversation was happening, saw the translucent Twilight eyeing a book on the table. He had grabbed it and was waving it around. The translucent Twilight's head followed it.
"You want the book girl?" he said, imitating the way Apple Jack talked to Winona, "You want the book?"
The Twilights looked a bit irritated at these proceedings, but they let it go on. Spike opened the door and tossed the book out.
"Go get it!" he said. Twilight flew, only slightly faster than normal, after it. After a few tense moments with her wings, she landed and walked out the door after it. Spike slammed the door shut and put his back to it.
"Whew, no offense Twi...s... but two of you is more than enough!" Spike said.
Discord chuckled. "Inspired, my draconian friend!" he said, "Now that we have some space, would anyone care for some tea?" A teapot materialized in his hand, along with a saucer and teacup. The saucer was already full of tea, and when he tipped the pot, it jumped into the cup. Fluttershy giggled lightly.
"Might as well," Sunset said, taking the offered saucer, "I'm all for experimenting, but if you could fill us in on exactly what's going on, it would save a lot of time."
Twilight H looked slightly crestfallen for a moment, then brightened. "Oh, so like a lecture?"
Discord considered this for a second, then snapped his claws. His living room transformed into a school room, with the four ponies and one dragon seated at desks around the room, with other Discords filling in the space. The Discord at the front of the room was waering a tweed jacket with patches on the sleeves. Those in the seats varied. One had hair with pig tails, one was wearing overalls, one a chearleading skirt, and so on.
"Psst, Twilight," said a Discord wearing a letter jacket, "Could you pass this note to Fluttershy?"
Twilight P blinked and levitated the note with magic. She moved it towards Fluttershy's desk.
"No passing notes in class!" Teacher Discord said, snapping a ruler down, "How would you like it if I read it in front of the class?"
"Oh! Please don't!" Fluttershy said, shrinking back, "It might be embarassing."
"What?" Twilight H asked, a bit dumbfounded.
"It's Discord, you get used to it," Spike said, unphased by the proceedings.
"Dear Fluttershy, do you like me? Yes, Definitely," Teacher Discord read off. Fluttershy and note-passer Discord blushed furiously and sunk back in their desks. The note itself, which Teacher Discord held up, was decorated with little hearts.**
"Discord!" Twilight H yelled.
"Oh, fine," Teacher Discord said, snapping his claw. Instantly they were back in the living room, and there was only the one Discord. Fluttershy was still hiding her face with her wing.
"I suppose we can do this the boring way," he said, taking a seat in a large backed chair and holding up a saucer with a cup of tea in it. He crossed his legs and took a sip.
"Now then, I suppose we should start from the beginning..." he said.
*I'm assuming they hadn't met... I didn't have the special on hand to check.
**Gravity Falls reference.