#soldyssym

To get the poly ending, you have to:

Do not date Dys when relationship with him reaches 100

But do flirt with Dys when the options arise otherwise

Do flirt with Sym, but do not date him until after this process

Let Dys blow up the wall or the medbay

Set Sol's relationship type to polyamorous (I usually just date Marz and Rex simultaneously then break up with them).

The year after Dys blows the colony and runs (which will be the next month, if he blows the wall during Glow!) Dys will be a boss event in the Wresting Ridges. This dialogue tree is usually where you either convince him to return to the colony, or help him upload to the array.

What should happen is, Sym will "accidentally" (on purpose) call Dys 'beloved' or similar, and Dys will confirm he and Sym are dating. Then you'll get to talk to Sym or Dys individually (you can go back and forth as many times as you like).

Normally, you would have to ask Sym to send Dys home with you. However, because the game remembers you flirted with Dys but didn't date him, you will have a romantic dialogue option to ask Dys to return to the colony with you as well.

Following this tree will establish that Sym is okay with sharing, and you can then take Dys back to the colony.

After this point, you can also start dating Sym.