In the original Age of Empire 2 (never played the HD version) there exists an undocumented "shared" multiplayer mode. When you choose same color, team and civilization for two teams, they can control the same units, have the same buildings, share the same resources etc. I.e. they act/appear as one player in the game, that can be controlled from two persons (more should be possible, but I have never tested).
It would be pretty cool if this is possible in openage too.