Am I correct in presuming that you are talking about a 2D sidescroller? If so the base engine of FOOGS would suit you. Also as part of the DogFight contest I have added SDL_Net support to S2DL so adding in a second player to foogs will be pretty straight forward.

If you want it isometric instead just wait for my DogFight Entry as it will already support two players and with a bit of tweaking should be able to support more players.