It's not necessarily irreversible separating them, but given that they are fully adult and not in their first or second winter, the chances of getting them back together would probably not be great. You could try 'time outs' on the aggressive degu, where you isolate him for a few hours with the hope that he will associate the isolation as a consequence of the aggressive behaviour. Have you got a sense of whether the battle is for dominance or if they are trying to eject someone?
It's battling for dominance in my opinion. Comparing to what the internet says it's dominance paired with the breeding season. Doesn't look like they're trying to eject someone because they've never been a "furball of death" not even when I introduced a new cagemate.
I'll try to relax because it's getting slightly better.