It depends on the person and the training program. Two lessons in a day without breaks (short or long) may not be wise. Or, it might be exactly what the CFI ordered.

Military, university, and other type programs are more than one event per day, even if one event is a ground session. Part 141 and 61 can do this too.

I had a client that was hitting a wall in his training. The only time he had available was the weekends and 1-2 evenings during the week, for which he'd arrive exhausted, too tired to learn. I got him to being ready for a checkride signoff by working him twice a day on the weekends. However, when the checkride had to be retaken, I had him in the aircraft every available evening and 3-4 sessions, basically all day each day during the weekend. He didn't need learning, he needed practicing so that he could perform under stress, while tired, while being extremely nervous, and he passed the ride after two weeks of my "owning" his schedule.

It doesn't have to be that extreme either. I've had several students do a two-fer or a three-fer in a day, mostly because they really enjoyed the learning & flying, and we got to visit some neat places.