Had to test this game by myself, but I think I kinda understood the gist of it.
The idea of having a single weapon (and a single controller) for two players is neat and original. I really enjoy the controls of the crabs, always facing the weapon, that's fitting, pretty unique and yet easy to understand.
The possibility of knocking off the knife is interesting, but unfortunately, from what I tested, it also means that the weaponless player can become invincible just by spamming the trigger. That kinda breaks the whole game! Maybe you could add a bit of timing control to it (like if you hit the button when you're about to be stabbed you send the knife flying, but otherwise you don't)?
Also, overall I feel like the game lacks a bit of juice. Maybe add some visual effects or change the music when one player picks the knife? You could probably make that knifey crab a bit more murderous than it is to add some fun!
Anyway, that's a pretty interesting game, some more polishing could make really good!