Great stuff. Very nice juice and a creative idea.
I especially liked the idea of lining up the filled lines.
As for feedback:
- I'd avoid the mixels (graphics with pixels of different sizes)
- I think waiting for the player to choose a side negates tetris's inherent urgency, I'd find a gameplay mechanic that keeps the game going without waiting for a player's choice, such as moving between sides while the piece is dropping, or at least picking the side randomly for the players if they take more than 2 seconds to pick.
- Edit: I see now that it does pick for the player after 10 seconds, but I think that's way too much time, and there's no indication for it