If there's only one Sunset Shimmer, this is why I think that is (Also: if there are two, where I think the other is) · 4:06pm Feb 7th, 2019
This isn't my preferred answer to the question "Where is the other Sunset Shimmer?" but it has the virtue of being straightforward and working with what we know about how the worlds do and don't parallel each other.
Unnecessary Deep Background:
The portal has been opened before. We don't know how many times because we don't know how old it is. (The only thing to address that wasn't canonical in the first place and was eventually contradicted by canon.) "Over a thousand years" is a pretty safe bet, which would have it opening over 445 times. Do we really think Sunset is the only cross pollination in that time?
Of course, we don't know that it's that old, but for this theory to work it only needs to be about 20 years old. One cycle less than 20 years would still get the job done, though it would require things to happen quickly.
More (and more important) Background (on parallels):
Through unexplained means the two worlds parallel each other with more or less the same people in more or less the same relationships. They don't do it on the same timeframe, but it seems to work out eventually. We even see that with the mane six. In the absence of Sunset Shimmer, the human five would have met Sci-Twi at the Friendship Games and, seeing how she was being bullied, would likely have reached out to her (as there would have been no magic to distract them) and then you'd have mirror friend groups (also resulting in Sci-Twi becoming Celestia's student.)
Where the two worlds emphatically don't parallel each other is when the portal is crossed.
Sci-Twi didn't come to Equestria where human Sunset had broken up the mane six less-Twilight. The human Dazzlings have not tried to take over Equestria only to be stopped by Humanset + mane six. Princess Twilight has never turned into a monster who rips holes between worlds.
Not only has none of that happened, some of it can't. Knowledge of what happened in the human world would allow them to diffuse some of those things in Equestria.
Symmetry only applies when the mirror isn't crossed. Once the mirror is crossed, symmetry breaks.
The Theory:
The simplest explanation for why we don't see another Sunset is because there isn't another Sunset. As noted in the title, if that's the case, I have a theory on why.
We don't know what measures Celestia took to stop ponies from crossing the portal, but we do know that on the other side it is rather emphatically not guarded. Any time it was open, a human could stumble upon it and go through.
So a human did. And they became a pony. And they met another pony. And they had a relationship, and they had a foal, and the foal's name was Sunset Shimmer.
Because it involves crossing the portal, the symmetry between worlds is broken anyway, and the theory can stop there. There wouldn't be another Sunset because we've already seen that the parallels between the worlds don't cover that situation.
Note on the probability
For Sunset to have parents from both worlds all you really need is a human to stumble through the portal and not immediately bolt back into their world. That's not that unlikely.
The portal is outdoors in a public place on a surface someone might try to lean against. It wouldn't be surprising if several humans have come into Equestria accidentally over the years, with most of them making quick exits (and never telling anyone about it for obvious reasons.)
One human doesn't bolt, ends up in a relationship, and you get a Sunset.
And it can be any human and any pony. Every single combination capable of producing an offspring (whatever that means in the magical land of Equestria) would work, so the chances of it happening are astronomically higher than even the most likely human-pony pairing.
For there to be two Sunsets in this case, though, all of that changes. Only one pairing in all of two worlds meets the requirement, the portal in Equestria isn't in a place where ponies are likely to randomly fall through, so on so forth.
Without the parallel world symmetry in effect, there's no reason to expect couples to be duplicated in general, to have it happen in a case with so much working against it would be statistical miracle.
So that's my theory for if there's just one Sunset. She had one parent from each world, and --given that-- her having a counterpart would be so unlikely as to be nigh impossible.
All this has happened before:
I didn't realize it when I first thought this up, but I have seen the idea that Sunset's parents come from different worlds done. Once. It's just that literally everything else in the story Who's your Daddy? by Sporktacles distracts from the concept.
Also, for reasons that should be clear to anyone who is familiar with the story, it doesn't rely on the symmetry breaking nature of crossing the portal. It has a . . . different explanation for why there's no second Sunset.
I noted at the beginning that this isn't my preferred explanation for why we haven't seen Humanset. My preferred explanation is that there is a counterpart to Sunset, she's just not where everyone's looking.
My headcanon is this:
Once upon a time in the non-magical city of Canterlot, a homeless orphan discovered something impossible. She was curious, and it wasn't as though things could get any worse, so she walked through.
At first she was disoriented and confused. When she looked at her hands she went into an utter panic. Some ponies --real live talking ponies!-- heard the noise she'd made in her distress. One of them helped her calm down a bit while another went off to get "the Princess".
The Princess was the most beautiful thing she'd ever seen, a winged unicorn that towered over her without being intimidating. The Princess made the remaining fear and unease go away, and told the orphan all about the magical land of Equestria over hot coco.
When the Princess finished telling her about this world, she said that they needed to return the orphan to her world, surely her parents would worry if--
The orphan panicked again. It wasn't fair! There was nothing for her in the other world, this one was so much better. There she was nothing and no one, here she was a unicorn! They couldn't show her this and then just take it away. It w--
The Princess comforted the orphan. Without pressing for specific details, she asked the orphan about her living situation.
When she learned that the orphan was alone, homeless, and going hungry more often than not, she promised that the orphan could stay in this world, that she wouldn't go hungry, that ponies would care for her.
The orphan rejoiced. She would stay in the world of castles and unicorns and food and warmth.
The Princess found the orphan a home, and a family, and the orphan was an orphan no more.
Then the Princess returned to her castle. It was night when she arrived. The Princess looked upon her sleeping student. In the morning she would apologize; the time it had taken to help the former orphan meant less for her student. It wasn't helped by the fact that the Princess hadn't been willing to tell her student the details of what she'd been doing.
How could the Princess tell her student about the orphan? She had her name. She had her face. The Princess would not disturb her student with such information.
Still, the Princess couldn't do nothing. Of all the ponies in the other world, it was her student's twin who came to this one. It had to mean something.
After apologizing, the Princess would show her student the mirror through which the orphan had arrived, hopefully glean some sense of what was going on, and she would make more time for the student going forward. If their lives were as similar as their bodies, her student had suffered more than the Princess had ever realized.
Those are some good ideas. I was actually thinking that the human Sunset might live somewhere different, as we know that pony Sunset is pretty intelligent, and the girl has moved somewhere where her talents are able to improve even faster. Maybe within the everton idependant study program Twilight wanted to join.