ugh, beats me. I don't there is a way to guarantee. often enough, you even have members of the other team in your system/ side of the map.
I never tried it, but: you could start the game, explore the map and if you're not together, save the game. then host the saved game and switch places with the allied ai's (so, occupy your friends former place with an ai and have an ai slot next to you open for other players), so that you are together. no idea whether it works, it might.
sry, never had the situation lately, hope it works for you.