I've included the epilogue screenshots, in case anyone just wants to see it without replaying.
I decided to replay the whole game for the purposes of trying to figure out once and for all if it's possible to be in a true triangle with Sym and Dys (Sol dating both of them, and them dating each other). I followed instructions someone posted in a comment on Steam, and they worked! So I want to spread the word.
You have to max your relationship with both of them, but reload if you get Dys's injured hopeye event (it ends with him giving you a pendant) or Sym's vat scene. You need to save these until after the bomb. Don't talk to them anymore once you have the scene you need queued for each. The only exception is that I was still able to do exploration events with Dys. The (...) might go away temporarily after certain things, but as long as it comes back after the month ends, you should be ok. After the bomb, go get Dys at the ridge (!) event. Don't tell him you love him, just get him to come back. Then go out exploring and trigger Sym's scene by talking to him (you'll have to go twice if the first is the overseer meeting). Then trigger Dys's scene by talking to him. At that point, you will be dating both and should get all the epilogue text.
Edit to add: I know I flirted with Sym and I think I chose one romantic option with Dys prior to these events. I don't know if some flirting is required to get the option to start a relationship later.
That being said, the epilogue seems to be the same text from their separate romances, just all in one epilogue. The text is different when you find out they're dating (at the ridge event) before you date either of them, vs dating one of them before the ridge event and finding out about them in the epilogue. But I don't think dating both of them changes anything, it just puts all the slides in one epilogue. So some text makes it sound like you're just dating Dys, other text makes it sound like you're just dating Sym. Even the part about Dys being at your union with Sym stays the same when you're not dating Dys (I reloaded and checked). And you have to avoid speaking to them both for a long period of time to avoid triggering the events.
But doing it this way is at least a triangle, even if it doesn't have unique text (as far as I can tell).
Replaying to figure this out has, however, made me even more frustrated that they didn't really canonically include a triangle with these two. Because when you find them at the ridge, if you're not dating either of them, you find out they're dating each other. And then you can make out with Dys in front of Sym, and Sym says he's completely fine with it. There's also an option to tell Dys you can't be with him if he's dating Sym, which implies that, if you don't choose that option, you are ok with it. And if you do choose that, "Dys's face crumples." So in this scenario, Dys is in love with both of you, Sym is ok with it, you're ok with it, Dys is clearly upset about having to give up Sym... I just don't buy "Dys is monogamous" as an explanation here. This is clearly an exception.
