I enjoyed both the games but never found the hidden third one, which is a shame. I think this would have benefited from allowing both players to be controlled at once, like with one on wasd and the other on the arrow keys or something. I found with both games I just parked one guy on the A button and used the second one exclusively. Allowing them both to be moved simultaneously would bring some extra cool depth of the games.
I found the bullet noise in Galaxy Gunner to get a little grating after awhile, which was a shame because all the music and all the other sound effects were excellent.
I also really liked how if you jumped over the flag and then hit it from behind !Mario would moonwalk off the stage. That might be a bug, but it was a fun one!