While there are no retail releases this week containing co-op, there are 2 downloadable games and add-ons available.
First up is Assault Heroes 2, the sequel to another Xbox Live Arcade title with a very similar name. The game is a top down shooter similar to Smash TV, but with the ability to play on foot and in a vehicle. The sequel promises enhanced co-op, with a new option for cooperative specific weapons that will "rain destruction down on enemies." The game hits Xbox Live Arcade on Wednesday for 800 MS Points.
Next up is an add-on pack to another top down shooter, Rocketmen: Axis of Evil. The add-on is called Rocketmen: It Came from You Uranus and will be available on the PSN and Xbox Live Arcade.