[thejeff]

Where top here is as shown in the Army Setup screen, which is determined by unit ID.
Not position on the battlefield.

In battle, spells are cast in Unit Id order. Communion Slaves can cast if no Communion Master has already cast this round. So, if you're trying to put together a reverse communion (Masters casting path boosters so all the slaves get boosted and can spam spells) make sure the Masters are last.
If you're just trying to boost a couple of Masters to cast big spells, then order doesn't matter.
As several people have said, there is only one communion. All masters share all slaves. This is a good thing.

Mystics are generally good for reverse communions, since you can easily get +1S+2E+2F+1W and 4 reinvig on your slaves.