Neat concept, but the execution is a little iffy - the second character didn't seem to offer any advantage, I just had to make sure at all times that neither player was dying. If you're open to suggestions, and haven't already considered it, I feel like making this an "Ikaruga" or "Outland" style game could really be awesome - each player character can only beat certain enemies.
Aside from that, the art and sounds are really nice. I'm familiar with your videos (which are EXCELLENT by the way, please keep making those), and this definitely feels like one of your games.