Well, democratic Great Powers often find enough points of agreement and are politically habituated enough to compromise to cooperate fairly often on the international scene. Dictatorships, especially cult-of-personality ones, OTOH...any disagreement on policy becomes a slight to the dictator, and will not be forgiven. Frankly, for a close and lasting Soviet-Chinese alliance, you need either a common menace (the Fascist US and surviving Third Reich Alliance scenario, or, for a milder one, surviving S. Chinese KMT state backed by US nukes), a much more democratic version of Communism, or an actual merger of the two states (say in a China liberated from Japan by Soviet forces scenario) which would probably require a more liberal form of Communist rule to work even if Russian fears of the tail wagging the dog could somehow be overcome.
Bruce