At the beginning of the battle assign a hierarchical order to all the Liche Priests in your army. The army's Hierophant is always the most important Priest, and High Priests are always more important than Liche Priests, but apart from these two rules you are free to choose any order you like. Write down the hierarchical order to remember it during the game.

During your Magic phases, the Priests will always perform their role in strict order (ie, do everything they intend to do in that Magic phase: cast their Incantations, use any Bound Spell items, etc from the least important Priest to the Hierophant, in a ritual that has been repeated for scores of centuries.

So always remember to do everything you intend to do with a Priest before you pass to the next in the hierarchic order: once a Priest has finished his actions and another has begun his, there is no going back.