Would the racists Japanese ever condescend to ally themselves with an inferior people (the Chinese)? I guess one could see them using the Chinese as cannon fodder.
I can see "Jingwei's" puppet government going along, but the alliance would be short lived and you have to wonder how motivated the Chinese conscripts would have been. Does this invasion occur before Barbarossa? If so, does Hitler sit back and allow the Sino-Japanese alliance do the heavy lifting, allowing Stalin to still fight a one front war, or does he commit his own troops to the mix?
If the invasion occurs in '39 does Hitler move on Russia earlier? -- What then does that do to Hitler's plans for France? -- If it's a go as early as '37 does Hitler get left out altogether?
Going with an early invasion ('35 to '37) as an off-shoot out of Japanese occupied Manchuria, Stalin, against such great numbers, would have found himself having to fight a scorched earth policy in reverse, retreating westward would have allowed Stalin to maintain control of his industrial centers and oil fields, and without German intervention (Hitler would not have been ready) would have been able to fight the Sino-Japanese forces to a stalemate.
The poorly trained (and likely unmotivated) Chinese armies, by shear numbers alone would been able to inflict early and devastating defeats the Soviets, but as with the Germans later on these early Sino-Japanese victories would eventually give way to a protracted war and the Chinese would not have been able to maintain the war logistically; the Sino-Japanese alias would have become strained and then broken. The Soviets would have prevailed.
On the other hand, given a later invasion, one as a joint effort with Operation Barbarossa (summer '41), it may well have proven disastrous for the Soviets; but the Japanese would have had to push back Pearl Harbor at least one year if not longer, and that opens up all kinds of possibilities for the Americans. (Either continued isolation or full commitment to the European theater.) If it is the former, the USSR would likely have ceased to exist, if it is the latter, Hitler may have found himself at bay in the west as early as '42, certainly by '43, and with full commitment the Americans would not have been asleep at Pearl and already in full war production.
But to repeat, one has to ask, with the later invasion ('41) occurring after the Rape of Nanjing could a defeated/puppet Chinese government be able to field a motivated army along side the hated Japanese? And then I go back to my original question, would the racists Japanese ever condescend to fight along side Chinese troops?
I don't mean to be disrespectful (as you can see I enjoyed trying to answer the question) but (IMHO) Japanese racism makes this question come close to being ASB. I don't believe the Japanese would ever ally themselves with any Asian sub-group.