I've been playing a variety of multiplayer combinations (2vs2, 3vs2, 1vs2) by PBEM for several years and it's been very successful.
Each team creates a shared Dropbox folder for their own orders. Player 1 on Team A does their orders, saves the file, copies it to the shared folder and notifies Player 2 on Team A .
Player 2 copies the file to their machine, gives their own orders and presses the button. Player 2 then copies the file to another Dropbox folder (shared with the Team B) and sends an email to let Team B know it's their turn.
It's best to agree house rules with your opponents prior to start and you can work out your own team's unit allocation and chain of command to suit your needs.
It's not hard to do and it adds a whole new dimension to the game experience.
If anyone is interested in playing with this method then please do get in contact. My buddy and I are always looking for new people to play against.