I don't schedule 2 flights per day unless one is solo and the other is dual. Most students can't handle 2 flights of 1.5 hours of dual unless they are CFIs are used to flying a few hours a day. I finished an MEI applicant last weekend, we flew two 2 hour flights in one day and he did fine.

I agree that students are customers but we as instructors are supposed to guide them to reach their goal. If the student hits a learning plateau after an hour, there is no point in doing another lesson and continuing the flight.