If it were practical to separate them, it should really be up to them, once they're adults, but if I were in that position, I could see wanting to be separated

If it were practical to separate them, it should really be up to them, once they're adults, but if I were in that position, I could see wanting to be separated for physical safety, if nothing else. Two bodies connected only at the skull seems extremely dangerous to me. For example, if one of them trips and falls, it seems like that could easily break the neck of the other.