I'm posting this in the "Military" forum to discuss the military aspect, versus any political aspect, of such an event on the United States. By "coordinated, joint attack" it is simply Russia and China's planned, coordinated, and commenced at the same time type of plan for both countries to launch some kind of military attack jointly on either U.S. forces, or key U.S.assets, at their weakest points.
It could be a symbolic provocation of some sort that would not lead to the loss of lives or property, such as suddenly surrounding and "locking U.S. forces out" of an important, or just symbolic asset of the U.S..
Perhaps most likely to occur would be the closing of some navigable body of water somewhere by their navy. And this is much less likely from Russia, as their navy has been long neglected, many of their ships don't even run on diesel fuel, some coal oil mixture unique to them. They barely sail.
Where would these events, geographically, be most likely to occur? And what assets stand most vulnerable right now?
Perhaps its not a good idea to get specific on any location, a general area is fine, especially if you have knowledge of any particularly specific or sensitive locations.