I direct a summer theater which draws in about 80 youth from our community every year, and in that group there's alot of great talent, so for the past 3 years, I've had to double cast. I've found that a good rule of thumb to go by is to have them all there at one time and split the rehearsal time in half for each cast. That way you can block cast one while cast two watches (or supposedly watches). That way when its cast two's time to block, they've seen what you've already blocked, and they pick it up in half the time (and you don't have to go through the heartbreak of explaining it in detail twice.), and at the next rehearsal, I let cast two go first while cast one watches and so on...