Here's the map I'm thinking of using for the CTW campaign:
I'm thinking the scenario will start off with the Communists fleeing Shanghai to go through warlord (barbarian) territories until they reach Yan'An.
Each turn, the Nationalists will put armies into territories they own but will not engage the Communists directly. While that's happening, Japanese armies will enter and seize coastal areas.
It should take the Communists about 5 turns to get to Yan'An, at which point they can begin diplomatic dealings, e.g. talk to the Soviets or the Nationalists. Eventually they must accede to an Alliance against the Nationalists.
This then gives China two attacks per turn against Japan (Nationalists and Communists each get one) which will eventually drive Japan's armies off the mainland.
Once that happens, it's a mad scramble to declare war against each other, with the Communists having a lot of cheap units and the Nationalists with more artillery and planes.