The idea
i now came up with is that you play together with your team mate, but can't help that good when it gets attacked by the enemy, because you got only a tiny passage to its base.
Each team mate has its own wider passage to the enemy, so everyone may attack on their own, BUT you can help your team mate if you are patient enough to get your troops through the tiny passage.
Plus, when you beat one enemy, attacking the other through the tiny enemy passage is not that easy as they have an easy game destroying your whole troops, because only 1-2 units can go through the passage side by side.
Of course this map could have up to 10 players with that concept.
EDIT: Some cosmetic improvements. Now it doesn't look that boring anymore.