I love the concept even though I didn't have anyone to play with right now, I can imagine how good this is in co-op. However I enjoyed it too much despite playing alone xD. Good job on keeping the difficulty on high level, it makes the game more challenging but in my opinion the hitbox should be smaller.
I believe developers don't usually make (same keyboard) co-op games because of the risk of getting low ratings from people that don't have someone to play with even if the game rocks. So grats on taking that risk and making a nice game.