Hi! First off, thank you so much for playing my little game.
The objective of the game is to play against your friend and get the opponent to place a piece at the 'top' of their side of the screen, much like regular tetris. Maybe I should add an in-game tutorial to avoid this confusion :)
The idea of a non-versus mode where you try to keep the playing field in balance is really cool though! I hadn't thought of that yet, but I can definitely see that work as a score-based (time survived or number of lines cleared) game.