Tag team army
Two players play in the same army and may select the same territories. If they both command on the same territory and there aren't enough to go around, in the game options it can either split it equally or randomly select one. They both earn and use cards separately (if selected in game options), and order priority/delay count against each other (and overcome double selected territories).
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powerneg commented
they could just both add and change orders together, changing each other's orders as if they were their own orders, i don't think many veterans are interested in this, but it would allow someone to "mentor" someone else into the game.