It depends on the student, I think. If they are studious and come prepared to each lesson, I'm willing to bet that they would get benefit out of a couple of flights in a day. If they are the type that show up having not cracked a book or thought about flying since the last time you saw them, then they are just going to be spending money, and not getting much out of it. There is also the option of doubling up time slots so that it's just one long flight vs. two short ones. This way, they spend less time doing the mundane things like flying to and from the practice areas, taxiing, etc. This can get exhausting though, so you have to watch out for fatigue both in yourself and in the student.

I did have some students that wanted to do this. For some I was glad to, and for others I did, but advised them to either ramp up their study habits to match their flight schedule, or vice versa.

I know that's not really an answer, but I suppose it's not really a black and white sort of thing really, either.