I haven't had a chance to properly look at how to do this so I'll leave it here and check if there are any replies tomorrow.

I would like to set up an invincible and perhaps invisible dud faction somewhere where no one can get to in order to co-op as the same faction in a sense. If the other player's faction falls, I would imagine they don't get to play anymore.

If the other player can still hang around after their faction disappearing then this is unnecessary.