Xybots(1987)

Xybots is a sci-fi shooter with pseudo-3D environments explored from a third-person perspective. The game can be played by one or two players; the two-player mode takes place on a split screen. The hero(es) must move through a series of Gauntlet-style underground mazes, killing the bad guys and collecting health and other bonuses from flying saucers. It's designed to be played co-operatively, although bullets from one player can harm the other.