In addition to navies, there were joint army groups. However, this would be hard to implement as it raises the question of who controls the group. I do like the idea of planes landing on allied carriers though, since the carrier owner could control the carrier, and the plane owner could control the plane.

You can coordinate with another player to send your fleets at exactly the same time, having each stack protecting the other (even better than one megastack actually). Of course, both players need to be there if you can direction, but that's the difficulty of coordination.

Example here :

rpgcodex.net/forums/index.php?…32309/page-4#post-6677189

I would leave the game as it is on this front. I am not sure we had during WW2 air groups from the US operating from UK carriers or the other way around.

Your planes will not be able to land on ally carriers and vice versa