Batman Bros. Co-Op 1.2 is a fan-made hack or modification of a Batman game for the NES, focusing on co-op gameplay. The number 1.2 denotes a version update suggesting improvements or changes from earlier versions. The gameplay would involve two players cooperatively navigating levels and fighting enemies.  Specifics of levels, enemies, and mechanics would require further research into this custom version of the game.

Batman Bros. Co-Op 1.2

Batman - The Video Game (USA) (Beta)