“Twilight!” Trixie let out excitedly as she slammed a book down in front of the purple mare, making Twilight flinch in surprise, “You'll never guess what Trixie found!” She said as Twilight looked to her, a little glad that Trixie was being loud in Luna's library, instead of a public one.
“Trixie, don't be so rough with it!” Twilight replied as she checked the book's condition, giving Trixie an annoyed look, which wavered under the blue unicorn's smile. Twilight then let out a sigh before checking the book's cover, “The (mostly) Complete Spell Index?” She asked, prompting Trixie to point out a particular spell within. “'Body Swapping'? What about it-” Twilight was about to ask, but paused at Trixie's growing grin. “No. Not happening.”
“Aww come on~. Trixie's always wanted to know what it's like to be in Twilight's hooves.” She stated, and Twilight couldn't deny that she was also curious to know how it felt in Trixie's hooves. However, “I said no, Trixie. A spell like this can go wrong very easily-”
“The Great and Powerful Trixie will get it right on the first try!” She stated confidently.
“-And there's the part where we'll need to cast it again to return to normal.” Twilight finished as she raised an eyebrow.
“Well, Trixie will get it right on the second try too!”
“How can you be sure? I'll be in your body, and you'll be in mine. What if our magic is swapped too?” Twilight asked, before her eyes widened in realization, “Actually, I guess it doesn't really matter.” She stated as she returned to looking through her book.
“What doesn't?” Trixie asked, trying not to sound upset, and failing.
“Sunset could cast the spell, but your magic isn't-” Twilight began, before hesitating, “Well, it's a taxing spell.”
“Trixie knows that, Twilight.” Trixie replied with a deadpan expression. “But if we cast it together-”
“No, not happening.” Twilight replied as she put her nose closer to the book, trying to keep Trixie outside of her peripheral vision in a vain attempt to avoid what was coming.
“Pleeaase?” Trixie asked as she made a face that nopony could say no to. And even though Twilight couldn't see her friend's face, she could picture it in her head. Those big unassuming eyes, almost on the verge of tears... Twilight let out an internal sigh as her face fell into her book.
“Fiine.” Twilight let out, slowly turning her head to give Trixie a hard look, “But just this once.”
“You're the best, Twilight!” Trixie let out, giving her friend a brief hug.
The two of them then looked over the spell, committing it to memory, before giving it a go. “Alright, Trixie has this, just add your magic to hers and we're good to go~!” she said as her horn started to glow, wrapping them both in its aura. Twilight hesitated for a moment, before calling on her own magic, and letting it feed into Trixie's. “And a three, two, one!” Trixie shouted, before there was a flash, followed by a big poof of smoke.
“Tr-Trixie,” Twilight coughed out, noting that she still had her own voice, “Did you really need to add smoke?” She asked as she appraised her hoof, seeing a purple coat instead of a blue one. And why can't I feel my hind legs?
“Smoke must always be added!” Trixie stated, almost sounding insulted. “Huh, that's weird. Why is Trixie still Trixie?” She asked, prompting Twilight to turn to her friend's voice, immediately falling down for it. “Ow!” Trixie let out, and in a flash of Trixie's magic, the smoke dissipated, making their eyes open wide as their mouths fell open. The top halves of their bodies were connected at the torso.
Twilight could see, a few paces nearby, that the bottom halves of their bodies were also connected at the torso, which had also toppled over.
“T-Tr-Trixie!” Twilight stuttered, her words failing her at how upset she was with this predicament.
“Well, the spell worked.” Trixie let out sheepishly as she rubbed the back of her head.
“Yes, I can clearly see that!” Twilight let out, making Trixie wince as she gave an apologetic look. How are our bodies even working!? Twilight wondered as a simple diagram of their respiratory system came up in her mind's eye, placing a questionmark where their bodies met. A questionmark preceded by the word 'magic'. “Look, let's just change back, before somepony sees us!” Twilight added as she glanced to the library door. Her heart then skipped a beat when her eyes met with Luna's Bookkeeper, Sir Tell Tale. The room seemed to freeze as he stood there for a moment, before shaking his head, turning away from the library.
“Huh?" A young female voice let out from behind the stallion, "Papa, why're we turnin' around?” The filly asked of the stallion, who immediately moved his tail to block his daughter from seeing what lay on the library's floor.
“Don't worry about it dear, we'll have to meet Cadance at her room instead.” He stated as he ushered his daughter away, “A couple of mares are fooling around in there, and it'd be rude to interrupt.” He added, getting a confused look from the filly as Twilight felt her face flush.
“Nopony's fooling around!” Trixie shouted as her horn glowed, “We just wanted to know what each others' bodies felt like!” she explained, making Twilight's face redden further before slamming her head against the floor.
“Trixie!” Twilight let out, glaring at the smiling mare, “What if he believes you! He might throw us out of the castle. We'd no longer be Luna's students! We'd have to leave Canterlot out of shame and become wandering unicorns with no name that-” she threw out, her face still flushed as Trixie put a hoof to Twilight's mouth.
“Don't be silly, Twilight, nopony would throw us out like that.” She then nodded to herself as she tapped her horn, “Besides, Trixie made sure they wouldn't hear what Trixie said." She admitted, getting a sigh of relief from Twilight. "Now, Trixie has to agree that you're right, we should probably change back to normal." She then let out a disappointing sigh, "But, just what went wrong?” She briefly wondered before shrugging, “Oh well, the spell worked, and that's all that matters. Better luck next time, Twilight!” She finished, in a tone that made it sound like the whole fiasco had been Twilight's idea.
“Trixie, need I remind you that this was your idea.” Twilight replied as she rubbed her temple.
“What, Trixie's?” She replied as she turned to the side, holding a hoof to her chest while wearing an innocent face.
“Yes, yours!” Twilight said as she tried to stand on her hooves, using her magic to pull Trixie onto hers. “Now, let's grab our flanks and get back to our original selves!”
“Alright, Trixie'll grab yours, and you can grab hers!” Trixie quickly replied, making Twilight raise a hoof, her mouth open to spit out a rebuttal. However, she found no words to offer, pausing before putting her hoof down, her shoulders sagging as she let out a long sigh.
“Sure, whatever,” Twilight replied with a deadpan expression, “let's just get back to normal.”
“Umm, about that...” Trixie stated as she looked to the empty library. Twilight's ears then fell when she picked up on Trixie's hesitation. “Where'd our tail ends go?”
“They're gone? But they were just here!”
“Huh, oh well," Trixie said with a shrug, "guess we're stuck this way~!” she stated, sounding the least bit bothered.
“We are?” Twilight stated as she adopted an insidious look, “Well then, I guess that'll make it easier for me to drill in all those history and math lessons you like to skip out on.”
“W-What!? No! Not history!” Trixie replied, almost looking green from her friend's suggestion.
“Especially history! So, unless you wanna be the pony with two heads for the rest of your life, let's get to finding our flanks!”
“Okay okay.” Trixie replied in a defeated tone, “I wonder what it'd be like to be the pony with two tails.”
“Trixie...” Twilight said as her brow furrowed.
“Y-You heard that? I mean-, Just kidding~,” she quickly replied with a forced smile before her horn started to glow, “Let's see here, our flanks couldn't have gotten far-” She began, but was interrupted by a loud pop and a flash of emerald light as Sunset appeared in the library.
Twilight let out a painful groan as Sunset's eyes fell on them; Trixie, however, let out a hearty gasp. “Hey Sunset! Care to join us!” She asked, causing Sunset to blink, before she scrutinized her conjoined friends for a moment, shaking her head.
“While I do like both of you,” she began, “there are some lines that must never be crossed.”
“Lines are made to be crossed!” Trixie stated, not missing a beat as she adopted a sagely look, “That's how you add and multiply after all~!”
“Says the mare who can't manage math.” Sunset replied as she raised an eyebrow, “Do I want to know how you two ended up this way?”
“Trixie wanted to try out a spell-” Twilight began.
“-And Trixie got to try it out!” Trixie finished.
“And what happened to your ... flanks?” Sunset asked, trying to conceal a small smile as she did.
“Oh, those? They went that way!” Trixie replied confidently as she pointed towards the back end of the library, getting an annoyed look from her conjoined 'twin'.
“You knew where they went!?” Twilight asked, toppling them over when she tried to raise her hooves in protest.
“Owie~” Trixie replied, before giving a small nod, “Trixie's magic felt another pony's magic grab our tails and pull 'us' away.” She explained as Twilight's ears fell.
“Somepony else knows...? This is the worse! How can we be Luna's students after this?” She asked as she and Trixie were covered by Sunset's magic. Their eyes then widened as they were assaulted by weightlessness, being levitated to Sunset's side as she gave Twilight a reassuring look.
“If you two get kicked out, I'll ensure that Spike and I send you postcards. You can expect them to say something like, 'Wish you were here' on them.” Sunset stated as she gave them a sarcastic smile. “Calm down Twilight, you're worrying a little too much over this." She said, looking Twilight in the eyes, prompting the purple mare to let out a sigh before giving a small nod. "Good. Now, let's get the two of you back to normal.”
“Sounds good!” Trixie let out as she pointed, “That way!” And with that, Sunset carried her friends in her magic as they went further into the library.
“Hey, wait a second, this way leads to-” Twilight started as panic lit in her eyes, her words dying off when they passed the last bookshelf that separated them from a particular desk.
The three(two?) of them paused when they saw that writhing on the ground in front of them were the tail ends of Trixie and Twilight. A floating feather was being used to tickle the sides of their conjoined behinds. Sunset, Trixie, and Twilight's eyes moved from the silently laughing double posterior to see Princess Luna sitting behind her desk. She was surrounded by unfinished paperwork as she rested her chin on her hooves, giving her students a very cheery smile.
“I have to say, right now I'm finding this half of you two to be the more pleasant half.” Luna stated as she pointed to the tickled victim before them, “She at least knows how to use her indoor voice.” She added as she tilted her head slightly. And this time, Twilight found that she wasn't alone when she flushed from embarrassment, seeing Trixie take up the exact same color as Luna's smile widened.
Author's Note
Inspired by the following pic, and ultimately by Lyra and Bon Bon in 'Do Princesses Dream of Magic Sheep?'
I saw that pic and couldn't resist writing up this silly little side-story.
This is my first one-shot; nice and short. I hope you got a least a chuckle out of it, and thanks as always for reading! 
