I've not seen it anywhere, but is there any kind of rule preventing the Communist chinese to stack; attack together, etc - with the Nationalist Chinese?
If that is not the case, what prevents the Chinese to just mass stack into Chunking, in wait for better times?
They may freely stack together and, as you amusingly write, all squat in Chungking if they wish.
Doing so would not only give the Chinese security, though, it would give Japan additional security, too, with no direct threat to Peking.
In the mid to late war, a great USA ploy is to have the Chinese hammer away at Peking with good hope of causing casualties that Japan can ill afford (or worse yet, risk losing that Resource to China).