I really love this game, and I agree that the art aesthetic here is one of my favorite!
I really like the combination of puzzle solving, adventure and exploring, and fighting.
A lot of times I kept forgetting one player or the other and confused like "why can't I move?" and then I imagined the other guy going "Hey, I'm right here buddy! Did you forget about me??"
I also loved that you could go on running water and have one on land that can kind of help you stay put.
I definitely spent the longest playing this game over any other I've tried so far. You should consider submitting this to Newgrounds especially if you're doing any more work on it! This is totally front page material right here.
My only real complaint is I wish the key to attack wasn't shift since attacking a bunch of times on Windows means I get the "Do you want to enable sticky keys" popup...