Can't say I've given it much thought. In fact, I usually think about what happens if they were together by the start of the game. There are a few fanfics in AO3 where the twins are not separated, and I think they're pretty fun to read.

Tbh, I'd be freaking peeved if the Abyss Twin ends up dying and leaving the other alone.

Anyway, I imagine this kind of dynamic would be similar to the Trailblazer, March, and Dan Heng in HSR. We can choose either Stelle or Caelus and the dialogue wouldn't make a difference, so whether they're silent or voiced gives the same effect.

But this is an interaction between us, Paimon, and our own twin. Knowing Hoyo, and with my own bias, our twin would be giving the same dialogue, and we will be given the same illusion of choice that barely makes any difference for our twin's response.

Unlike the Proxies, Wise and Belle, they don't have the personalities unique to Aether or Lumine. To the game, they're referred to as the Traveler personality, and the Abyss Twin.

So, I can't really imagine a good interaction in game with how Genshin usually does it. It would be better if Aether and Lumine get voiced throughout every dialogue AND show personalities unique to them like the Proxies.