Actually I am pretty sure that I once had masters and slaves doing things simultaneously. I think it had to do with the order in which spells were cast...(master before/after slave)
Slaves either help masters to cast spells or do something on their own. So if your masters are scripted to attack, slaves will follow their orders. If you have few masters and a lot of slaves, some slaves (those who're not helping masters) will follow orders too. But in this case I'm not sure which ones will be helping masters and which will be following orders...
alexti: why would some slaves not help a master and others do help? Perhaps because you only get a power boost at 2, 4, or 8 slaves, so if you have 1, 3, 5, 6, or 7 slaves then some of them will not bother to help?