[thejeff]

Slightly expanded answer. All communion masters in a battle (on the same side, of course) make use of all communion slaves.

You can have as many of each as you want, but there is only one communion. This is generally good. Instead of 2 communions of 8 slaves, each boosting their master by 3, you get one of 16 slaves, boosting both masters by 4. Slave fatigue is also further reduced and either master's buffs apply to all slaves.