[Tuna-Fish]

What happens if there are two slaves and one passes out? Does master lose empowerment? Does all fatigue go to the "awake" one and the "sleeping" one gets none?

Passing out doesn't matter, they stay as a part of the communion until they die, that happens soon after 200 fatique. Take care of the fatique removal for your slaves!

What happens if there are two slaves and both pass out? Does master lose +1 path empowerment?

noup, only after they die.

What happens if 4 mages cast communion slave and 2 cast communion master? are both masters empowered by +2 path?

Yes, but it is likely that your masters will kill the slaves due to too high fatique. Casting earth power is a good idea, it gives fatique removal to the master and all of the slaves. In big battles casting some self-defence buffs with a master is also a good idea, to protect the usually very fragile slaves.