Let's say that if both armies are trapped in a mine after a collapse, they will remain there until someone digs them out. Each turn, a portion of the army will die from hunger (or something else), so it's important to dig them out before it's too late. When digging them out, there could be two scenarios:
1. An army is dug out (one of the two) by an ally/army of the same color. Both armies then fight the enemy army in turn.
2. The army is "neutral." Whoever digs them out fights both armies, and the winner is chosen from the mine.