I think that Wil and Dark use their old names as both insults and terms of endearment, like if they're in an argument and Wil calls Dark Damien/Celine - although Dark is just a combination of their worst traits - and Dark uses William, it's said with so much disgust about what could have been. But in an intimate moment, uses those names is a memory of how far they've come and the fact that they're still there together, and the aspects they love about each other are still there in some way
im sorry i also just. i cant stop imagining dark just being vaguely annoyed and calling wilford “william” in THAT tone of voice.
you know the one, like with the exaggerated emphasis on the name
“Well maybe you should have done the thing, william.”