She's the one having issues with the living arrangements; I don't really understand why she doesn't remove herself from the situation rather than trying to coerce you into doing the same.
Edited to add: Whoops... Missed the whole "legally married" part the first time I read this (reading while sleepy is always a risk). Much as I don't want to rehash the "couples privilege" thread, I can understand why this is a bigger deal for her to just move out than to ask you to do the same. I *do* agree with NYCindie below that she should work at contributing toward the separate residences, rather than expect him to provide for you both. There's a part of me that thinks that she should *know* that he can't financially do both, so I'm suspicious that her asking him to do so is a backhanded way of getting him to break up with one of you... most likely you, since they're legally married. But it's early and I haven't had my coffee yet.
Why are you not in favor of separate residences (aside from the financial part) if it means a better working relationship all around? Is it that you'd prefer a "Poly family" arrangement, or that you wouldn't like the scheduling and time impacts that come with such an arrangement?