I don't know about other pitch control programs, but I believe that if you use some of the methods mentioned above, you can get enough isolation to use Melodyne to correct pitch. How close are the lead and tenor to the studio? Can they go an make some test recordings to see if they can be successfully manipulated? How big is the studio? If you have enough space (8-10' is plenty) I'm sure that singing close to cardioid dynamic mics will give enough isolation. I've done it with full band with drums.