[magnate]

I think the important gameplay issue to solve is allowing two allied armies to fight an enemy army *together*. I don't particularly mind whether you have a straightforward MoO-type diplomacy system or some kind of vassalage/pantheon system, but the whole point of being able to work together is to defeat people you couldn't defeat on your own. Sequential attacks on a powerful enemy are a lot less effective than attacking together. In fact now I come to think of it why limit the design to two allied armies ... a certain famous Middle Earth battle comes to mind ...