3 faction battles in Warhammer?

So I'm playing a co-op campaign with a friend, with him as the Dwarfs and myself as the Empire. We're currently preparing to take a Dwarf Hold in the Black Mountains with two armies, one of mine and one of his.

I was just wondering whether it's possible to fight the siege with him attacking and me as reinforcements (so that would mean 3 armies from three different factions), because I haven't seen that kind of thing in earlier Total Wars.