I guess my only thought is "why wouldn't they?" In an RTS FFA custom game, the two less strong contenders are always going to at least try to give the appearance of a united front against the score leader.
Aren't they doing this before? They have recently knot their ties with each other right? This is not even a big news. It will be if USA joins them but on the other hand, they're both doing this to intimidate the states. It's an ongoing cold"ego"war among those superpowers.