Interesting. I think the flexibility might offer an advantage to the Axis, since they know what they need to do (and how the enemy will react to it) in advance. It depends how constrictive the rules are. If they’re loose… I’m joining with Stalin and smashing the world!
I’ve always said that the Japanese successes in the Pacific were entirely dependent on the fall of France and an isolated Britain. They were - much more than those of Germany - opportunistic successes. As time has gone on my feelings on Japan have changed from an underdeveloped ‘Germany’ of the east, to something more akin to an Italy of the east, with soldiers that actually wanted to fight (if that makes sense). Japanese successes were completely dependent on the Germans tying everyone up, whereas German successes had nothing at all to do with Japan (arguably, the actions of the Japanese made things more difficult for them).
In the game I think that the ability to re-strategise the war would benefit the Axis more in the merged game. But we’ll have to wait and see. It all depends how much equipment they have on the map at the start of the game, and how strong their enemies are.