When linked with 2 Gameboys and 2 copies of the game, the entire campaign in Double Dragon II can be played in co-op. This is a much more fun way to play this slow moving brawler.
Double Dragon II on Gameboy is also very different than the NES version. When A+B are pressed, they duck instead of the jump. Follow that command with a punch or kick to unleash a special move. And there are no bad platforming sequences since there is no jump ability.
All around, this is a much better 2-player mode than the original.