Just to clarify since this thread is most ancient, there is no co-op, the modding didn't work as the game goes out of sync too often. There was someone who claimed to have the source code and was going to mod it in but we've never heard from them since.
Closest you are going to get is to convert each map and put the opponents under the control of a human player so that you can team up i.e 2 vs 1.