I cannot think of a way the coop multiplayer would work if you can only control what player A controls also. What I was thinking on how it could work would be similar to maybe DF + DayZ.
You would start in an area with your settlers in a random location on the map or you can choose to start off next to your friends. Your friends cannot control your own settlers unless you give them “permission.” The settlers could also help other settlers that are allied with you to build certain structures but you’d have to order them for this. Everything would need to be tasked to help an ally but you could manage your own units.
I have no idea how this would work… I’m also thinking of coop multiplayer being similar to a Minecraft server where you can have 20+ people playing as individuals. Which would be awesome.