Please register for Total War Access to use the forums. If you're an existing user, your forum details will be merged with Total War Access if you register with the same email or username. For more information please read our FAQ’s here.
Currently you are not allowed to set up an ambush in close proximity to your co-op multiplayer ally, if your turn is first. The only way to do so is to have the first turn player set up ambush and then have the second turn player move to the first player's ambush stance. I feel like you should be able to set up ambushes when close in proximity to allies no matter what.