In an attempt to limit confusion while keeping the flow of the story as natural as possible, Alicorn Celestia will be referred to as the Princess or Princess Celestia, while Unicorn/Human Celestia will just be called Celestia. If it still ends up confusing please let me know and I'll try to come up with a better notation to differentiate them.
Princess Celestia and Sunset Shimmer found themselves just standing and watching the scene before them unfold. It had taken a few minutes for the Princess to finish venting years of worry for her former student, but the hug had finally come to an end, and the two had then caught sight of the other two ponies in the room.
"So this is weird." Princess Celestia said with a light chuckle.
"I imagine meeting a not-quite-right version of yourself would be pretty strange." Sunset agreed.
"Actually that's a bit more common than you would think, and not what I was talking about." Princess Celestia said with a wry grin. "I was talking about that." she added, gesturing with a hoof.
Before them, Luna had effectively pinned a shell-shocked looking High School Principal turned mythical creature to the ground and was vigorously rubbing her cheek against Celestia's. "Sister, look! She's so cute! Can I keep her? Please! I promise I'll take better care of her than that timberwolf!" Luna begged, while attempting to make her eyes look as large and as sad as possible.
Princess Celestia gave her sister an exasperated look. "No you absolutely cannot 'keep her', I'm fairly sure that would technically be slavery, even if she isn't really a pony, though that's just my assumption."
While somewhere along the way Celestia had realized this Luna likely wasn't her sister, the whole experience still left her a bit dazed. The comforting familiarity of her overly affectionate and goofy sister battled with the insanity of seeing her as a giant fluffy pony. Despite all that, Celestia still managed to snap out of it as she heard the Princess correctly point out that she wasn't a pony. "Wait, how did you know that?" She asked as she not-so-gently extracted herself from underneath Luna.
"This sort of thing may have happened before." Princess Celestia admitted with a sigh. "Speaking of which, what did you used to be?"
"Um, human?" Celestia half answered and half asked, hoping her other self would be less foreign to the term than Twilight had been.
"Ah of course, that would explain why Sunset is here." Princess Celestia said as several mental puzzle pieces clicked neatly into place. "Why don't we go and have a bit of a chat while Sunset and Luna get a chance to meet each other." Princess Celestia turned her attention back towards her sister and Sunset. "Despite having always wanted to, due to ah...circumstances, I've never had the opportunity to introduce the two of you."
Looks of sheepish guilt flashed across both the reformed villains' faces almost simultaneously, and Princess Celestia couldn't help but giggle to herself for a moment. "I think you both may find you have a lot in common." With that, she gave the two a quick wink and turned towards a door, gesturing for Celestia to follow.
Following the Princess' lead, Celestia found herself in a cozy tea room that seemed to have an unnatural number of windows and skylights that made the room feel as if it was filled with the midday sun from every angle. Princess Celestia gravitated immediately towards a large, well worn cushion, while directing her towards one of several smaller cushions. Celestia quickly plopped down onto one, grateful she didn't need to try furniture climbing yet again.
As she was getting comfortable, a pony in what looked like a stereotypical butler outfit converted to fit a pony quietly entered the room, deposited an enormous tea tray in front of them, and then just as discreetly exited. All in all it was much calmer and less shocking service than she had received in Twilight's castle.
Princess Celestia quickly swept two cups and the tea pot up in her golden magic and began expertly pouring two perfect portions of the steaming liquid. "If I know myself half as well as I think I do, I think you'll enjoy this," she said playfully as she daintily floated the cup into range of the unicorn's similarly hued magic.
Celestia smiled gratefully, trying to shake off the feeling that she was staring at a weird mixture of herself and her mother as she took a small sip of the fresh tea. Immediately her eyes widened in surprise and she found herself greedily gulping down the entire cup. "Holy hell that's good tea!" she exclaimed with a contented sigh, before cringing and looking up at her hostess. "I-I'm sorry, I'm sure that was very rude...I've never been in the presence of royalty before."
The Princess merely chuckled and began to refill the empty cup. "I'm fairly sure my own reaction was far more vulgar when I first tried it, I had a very foul mouth when I was younger." she admitted with a hint of nostalgia. "though I suppose you know that already. Speaking of which, how old are you?"
Celestia was somewhat taken aback by the sudden personal question, but figured there was no harm in answering when it was essentially herself she was telling. "Um...29," she answered casually after a moment's hesitation.
Princess Celestia gave her a strange grin in response. "Hmmm I see. You should get better at fibbing, you'll be doing it for awhile. Even after 3,817 years, I still like to lie about my age too." she explained with another mischievous wink.
Celestia's eye twitched at hearing the impossible approximate age of her counterpart as she attempted to fathom even a fraction of such a long lifetime. "You can't possibly be anywhere near that age!"
"Oh I assure you, it's entirely...mostly...true," Princess Celestia responded, giving her a truthful, if still somewhat deviant smile. "Though I very clearly remember being your age as well. I had such a temper back then...I miss it sometimes, it's hard to get riled up about things when you've seen them a million times," she admitted with a tired sigh. "Anyways, I'm sure you didn't come here so I could reminisce about my youth, what brings you to Equestria?"
Celestia took a moment to think about it, but realized there was no real elegant way to put it. "I just sort of...fell, and then ended up here," she said with a shrug. "Next thing I know, two of my students are explaining why everyone is a pony and why I can't go home. For the sake of my mental health I've given up on trying to understand any of it."
Again the Princess found herself chuckling in the face of behaviors and mannerisms she had long forgotten doing herself. "Well my little...me...that's probably for the best. Inter-dimensional portals can be finicky things and you likely won't be able to return anytime soon. I highly suggest you attempt to enjoy yourself in the mean time, and things will work themselves out when the time comes. Besides, what better place for a vacation than somewhere where you technically made all the rules."
The constant mischievous glint in the Princess' eye seemed to only strengthen as she spoke, and at the mention of the the dreaded word 'vacation', Celestia began to suspect that maybe she knew more about things than she was letting on. She considered trying to probe further, but at the same time she couldn't help but feel like being given cryptic advice from an all knowing immortal goddess was just something that is supposed to happen when you end up in a magic fantasy land.
"You've definitely been reading too many fantasy novels..." she muttered to herself, returning to sipping at the delightful tea.
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As the pair of Celestias walked out of the room, Luna and Sunset looked at each other with interest, both having noticed their identical reactions to Princess Celestia's words.
"So, what did you do?" Sunset asked carefully, hoping she was understanding the situation correctly.
Luna gave a strained frown at the question, never eager to discuss her time as Nightmare Moon, but figured there had to be some reason her sister had brought attention to it, so with a resigned sigh she answered. "I sort of...let myself get so jealous of my sister that I tried to murder her, take over Equestria, and plunge it into eternal darkness, only to get banished to the moon where I spent 1000 years plotting revenge..." she admitted, feeling the guilt wash over her yet again. "You?"
It was Sunset's turn to squirm somewhat, having only just stopped experiencing reoccurring nightmares starring herself as a murderous demon. "Well I...ran away to another dimension when the Princess wouldn't make me a princess, then I tried to rule over that place like a princess by stealing the element of magic, which then turned me into a demon with a zombie army..."
Both of them at this point were cemented in a cringe and had their eyes closed tight to avoid any judging looks from the other. As each of their stories sunk in however, both slowly relaxed and looked at the other curiously.
"Huh.." was all Luna could think to say, realizing for the first time she had found somepony else that understood what it was like to be feared and avoided for past misdeeds.
"Yeah..." Sunset was at a similar loss for words. Even though her friends and a few other students had forgiven her, quite a few still gave her cold looks and treated her like her old self. For once she felt like she was in the presence of someone she didn't have to prove her reformation to.
Both of them stood silently for a minute, just coming to terms with the slightly awkward but also slightly fantastic feeling of understanding. Finally, a rather foreign question popped into Luna's head.
"Soooo...friends?" she asked, trying and failing miserably to keep her desperate hope from showing in her voice.
"Yeah...I think I'd like that very much." Sunset said almost immediately, a small smile curling her lips.
Luna smiled as well, moving in closer to Sunset and wrapping a navy blue wing around her before moving in to whisper in her ear. "So...do you wanna go steal one sock from every pair my sister owns? I happen to know she has a rather large collection and likes to wear them whenever nobody is around."
Sunset's timid smile immediately spread into a rather predatory looking grin. "Yes, yes I do." Sunset may have been reformed, but she wasn't that reformed.
With what sounded like a miniature version of a supervillain's laugh, Luna grabbed Sunset's hoof and began leading her back towards the royal bedchambers, the two of them leaving just in time to miss hearing a shrill cry ring from the adjacent tea room, though had they heard it, it would have been impossible to tell which white pony was responsible.
"WHAT DOES THAT MEAN? DO I LIKE SOCKS? WHO TOLD YOU!?"