Well mfinley, it depends. On how dedicated you are as an instructor. Cats copy each other, hence the term "copycat." They will only learn as fast as the least-motivated, most-stubborn cat. If there is one litterbox still around for that one to use, then they all will use it and refuse to be trained. You would have to be very vigilant about stopping mistakes before they turn into full on kitty mutiny.
Log on to the group, Cats T-Training ( I think it is on y a h o o groups) and they have lots of resources for you about training multiple cats.
Best of luck!
Julie (animallogic)