I really like this ! The simple twist on the scrolling shooter genre is really effective in this case and makes a fun game ! I don't have someone to try it with me at the moment but i can already tell it must be really fun to play this with someone and break a friendship over non-coordination.
The art is nice but the only problem for me is that the enemy and the player have the same color palettes. It doesn't make them pop as much as they could i think. A good thing to correct that would be an outline or just a really slight hue variation so we can still understand that they're both of the same "color" but not on the same team.
The other detail that could be corrected is the font on the game over screen which is "smooth" when it could be the same 8bit font that you have on the main screen.
I'm being picky but overall this game is really cool ! Well done !