Hey, thanks for playing! Pianos have their equal counterparts not far that indicates which key should be pressed in order to open the door. All pianos must have the right key pressed. It was the least easy puzzle to convey, sorry for that.
No worries my friend, puzzle games are the hardest for jams since they're so difficult to playtest!
I would suggest changing level 2 to a series of rooms, with the first few having a single piano in, and then the final room can be two pianos. I think that would make it much easier to teach the player what they do, because right now you don't get any feedback unless both pianos are on the right key.