Note: "if two opposing armies with the same support move into an undefended province, they fight a battle there. The winners of the random selection will fight as the defenders, but if they lose the battle, they will retreat as attackers."
This applies to the case of allied kingdoms as well. If the move is made as a result of failed coordination rather than betrayal, then they can choose not to fight the battle, but one of them will still have to retreat out of the province.
Edit: on signups, we're still waiting for OL, Norsemen, RS and Einhärjar/Shieldings, since they indicated interest before. If they don't materialize, and nobody else signs up, then I'll probably send out a general invitation to all major EU clans so that we can get started.