Let both armies be trapped in the mine after the collapse until someone digs them out. With each turn, part of the army will die from hunger (or something else), so you need to dig them out before it's too late. When digging them out, there can be two scenarios:
1. An army is dug out (one of the two) by an ally/army of the same color. Both armies take turns fighting the enemy army.
2. The army is "neutral." Whoever digs them out fights both armies, and the winner is chosen from the mine.