In the official interview weekends they usually have stuff (tours, lectures, presentations, arranged dinners, etc.) that you might not get if you were to go at another time on your own...Of course some schools have multiple weekends too. Ask your schools if this is the case, if so rescheduling should not deprive you of those events. I guess you could try and visit both if they are very very close. Take into consideration how much your travel time will be.
So if this is the only weekend for both and you definitely cannot combine visits I say go for School #1.