I really enjoyed this, i loved playing it single player too - the multitasking just added that extra bit of challenge over a normal game, and i loved trying to force myself to play multiple characters at once. By the way, i thought the menu and the control screen were really nice, and in general the style of the ui was nice.
In terms of feedback, I felt like sometimes it was hard for the second player to actually stay attached the other player? I'd end up falling off even if i wasn't pressing any movement keys on the second character, then there was an ordeal to get back on top of the first player. Maybe some sort of locking mechanism would be cool to attach the players?
Also, past the first locked door, there's a segment where the second player needed to fetch green cubes without getting hit by the enemy, but the second player moves so slow! You could grab the opposite end collectable, but i died so many times trying to get the middle one. Maybe a ground based lock between the players where the first player could push the second one at a faster speed would be cool here?
Overall I loved it, and it's impressive work for a single person :)