I don't know the answer to that.
I do know if you try to be clever and have slaves/masters all casting every round, then by the end of the battle you will have alot of dead mages!
After killing several groups of 8 and 16 mages using communion slave/master, I give up on it!
I would just use communion slave/master in extreme emergency when you desperately need a certain high level spell and expect to lose the mages.
Fatigue = mage killer
