Where I think the other Sunset Shimmer is, the short version. · 7:12pm Apr 12th, 2019
Anyone who tried to read through my previous post on the topic likely noticed what an absolute mess it was. At the time I wasn't in the sort of mental state that allowed for either the control to not go all over the place, or the editing needed to turn an "all over the place" result into something that was readable.
So, now that I'm somewhat more coherent, here's another shot. No idea if it will work.
While my headcanon is different, the simplest immediate answer to the question is that there isn't another Sunset Shimmer. I say "immediate" because, while that's really straightforward in terms of why we don't see her, it raises the question of why there isn't another Sunset. A lot of people go to tragedy (or equally heavy places) to answer the second question. I don't think that's necessary.
What we see in Friendship is Magic and Equestria Girls gives us a pretty good idea of where the two worlds do and do not mirror each other, and there's one area that ranks above all others when it comes to breaking symmetry. It's travel. The parallels between worlds completely break down when someone goes from one world to the other.
If you couple that with the fact that the human side of the portal is out in the open where anyone can fall through just by trying to lean on a seemingly inviting pillar, then there's a really simple explanation for how why Sunset Shimmer doesn't have a counterpart: one of her parents was originally human.
Everything we've seen in Equestrian Girls says that if someone goes through the portal and starts a relationship on the other side, the "parallel worlds develop in parallel ways" rule is going to break down. Unless it's part of some "let's swap lives" arrangement, the odds of the world hopper's counterpart doing the reverse are basically nil.
So all you need to do is let the worldhopper be one of Sunset's parents, and the lack of her counterpart makes perfect sense. Her counterparts would-be parents were never in the same world, which would make it somewhat difficult for them to have a kid together.
Now, granted, most people stumbling through a magical portal into a world where they're a different species are probably going to get the fuck back out of that world as soon as possible (and then never mention it, for fear they'd look insane), but only one person in all of history needs to have a different approach for the theory to work.
It has to be in a specific timeframe, to be sure, but given the way that probability works in MLP, if a highly unlikely or improbable event does take place, the odds that it will in some way be relevant to the main characters (instead of, say, only having an effect upon a small village in Norway in the spring of 1632) are pretty much one in one.
I mentioned that that isn't my headcanon. My headcanon also involves someone going through the portal, but it's simpler.
I just figure that human Sunset came through the portal first. If her life sucked enough, leaving everything behind and moving to magical land of talking ponies (where she's a unicorn!) would be pretty appealing.
Once Princess Celestia got Humanset into a permanent home, she'd want to investigate why the only creature to ever come out of the portal looked exactly like her student. Part of that would be showing Canonset the mirror. From there, plot happens.
The end.
That assumes that, even though travel across the mirror isn't subject to symmetry, something is causing both Sunsets to swap worlds regardless. It basically rests on the idea that Canonset Shimmer belongs in the human world and it would make sense for her counterpart to belong in Equestria. All you need then is the handwavy idea that destiny is going to get people to wherever it is they happen to belong.
My own favorite explanation for Human!Sunset used to be that she's living with Mane!Sunset as identical twins. Unfortunately, canon has pretty much eliminated that possibility by now.
After that, I liked the explanation you picked in a couple of your stories: that she's run away from home. But this works too: she's not just run away but run away to Equestria!
Next question: Where is she? Why hasn't she shown up onscreen? Is it perhaps because Princess Celestia gave her a disguise spell and she's now got a mint-green coat and a fascination with sitting upright like a human?
(Okay, probably not: we've seen human!Lyra too. But there're a lot of other interesting unicorns whose human counterparts we haven't seen...)
There's a story with a similar premise than your idea, 'cept in this case both Sunsets cross the portal at the same time, leaving the other to pick up their respective messes: https://www.fimfiction.net/story/431902/second-sunset