I've heard that it's safest to get one done and then the other for a few reasons:
#1) If something doesn't set right on the first knee, and the patient complains about it, chances are the doctor will know how to do it better for the second knee, and maybe even be able to fix the first knee, thus saving the patient some money instead of going back with two wrongly-done knees.
#2) It is easier for the doctor. It will make it so he doesn't have to rush to get two knees done, therefore he will concentrate on the knee having more time to think. The more order the better, and for doctors, order is time.
#3)(Being an optomist) The patient won't have as much fear going in for the second operation, having already a knowledge of what will be done.
Patrick