I think this is reasonable enough. If two married people whose only filing options are MFJ or MFS both come to me for tax prep, I assume their goal is to achieve the best tax outcome for the family. If they are feuding and carrying on and looking out for Number 1 only, they are going to two different preparers anyway, just like they're going to two different attorneys.
I will say that If they tell me they are splitting up, and want to know about their individual liability, I have no problem figuring that. I also don't mind directing refunds to two accounts. But I assume they wouldn't both be in my office if they weren't interested in working together.