Well in the past, it was just one side against the other but the players all get teamed on one side and the AI are all on the other (as far as i know anyway, someone more knowledgeable feel free to jump in here). What i think we're talking about here is having allies and soviets on the same "faction" as far as the engine knows, with structures and units from both, all fighting against the Yuri AI. That about right?
If this were possible, it would be awesome but i'm not sure it can be done and still maintain the Allied and Soviet faction identities. I've seen some topics similar to this about having 3 factions and i think that poses some difficulty for the engine. But if say....all the units and structures could be crammed (h4x0r3d) into one "faction" (allies for example) for a special co-op or rather collaboration mission against the other Yuri AI faction (soviet), im sure it would be extremely popular and work well too! Assuming all the stuff can be crammed together like that