Actually, CoM is almost completely unnecessary for understanding KH2's prologue. Here's what we learn in KH:CoM:
-Introduces Namine; no, you don't actually learn anything about her.
-Explains why Sora is in the egg thingy (Just assume "A wizard did it" and you can move on from there.)
-Explains what happens to Riku and King Mickey within that time frame (KH2 more or less summarizes it well enough. The only thing left out is seeing Riku develop from basically being pushed around by the villains to him just sort of giving them the finger and kicking their asses, which is actually not half badly written...actually, Riku's side is pretty well done overall, just requires going through Sora's boring as hell mode first.)
-introduces Orgy 13 but in a way that's basically "they're cryptic evil guys." By extension, it also explains where there are only 7 of them in KH2.
...yeah when it comes down to it, that last point is really why KHCoM exists. Having to fight all of the Orgy members in one game, some of them multiple times, could be a bit excessive, so they basically used KHCoM to deal with half the characters then, and have an excuse to write them out of the plot before KH2 happened.
Frankly, if you want a game that actually bridges KH1 and KH2, and actually shows a semblance of significance, 358/2 Days is the one to play. Shows you what actually happens in the year between the two games, and from Roxas' point of view, so we actually learn about the Orgy 13 from the inside, and the game is not badly written to be honest.