Very cool game
I really liked the idea, it is definitely fun controlling two characters, although my brain quite often tries to control the character on the left of of the screen with my left hand and vice versa!
I always liked controlling two characters at once, I remember attempting a few shoot 'em up games like that, very fun and is a little badass too :P
Difficulty curve was kinda whack XD But I suppose with this kind of game it would be really hard to get the curve correct. You should probably take a leaf from Portal 1 and 2's book, they introduce the player to elements one at a time and then mix them up, but in this game some levels that had unique-ish elements were just thrown in there.
And the second thing: Where was down advertised as the switch button? Lol. Took me ages to get past the first level because I didn't know the diagonal thing with the red end was a switch, I thought it was a bloodied spike, and I didn't know down switched switches.