This method works but usually one of us is the server, we haven't tested on both being clients, the server guy gets on a plane then the other guy CREATES a plane and after that he can go back to map (Hit ESC) and click on friends plane and join him. We did that to fly He111 as bomber/gunner crew.
Also with this method you can start coop style missions also. After you start the mission the "server" jumps in the leader of the flight you have created, so none of the AIs will fly away, the client guys CREATE any plane and then back to map and choose AI plane of the grp
