I think it is also poor taste to ask your prospective department to provide you with a single room just for you and your spouse. All of my interview accommodations included two prospectives in a room. I would agree with @TheDidacticStudent to book your own room, maybe conveying that to your point of contact and seeing if they come back by offering you a single room. Otherwise, it's a cost the school would have to pay for, and I don't think prospective students have that kind of leverage. It would be different, however, if you were a superstar professor in the process of being poached.