Would the game declare it a draw? Start over from the beginning? Or just do nothing and wait for one of the players to disconnect? Anybody interested in trying to find out?
I'd be interested in trying to find out. Actually, it wouldn't be that hard, but you would need to use an L or J piece to do it without leaving extra blocks. I'm fairly sure that i've seen a video or something of this before. If both players laid down blocks down the centre they would stick where they collided, wouldn't they?
The only games I know of where more than one tetromino falls into the same well at once are 1. Tet?is by Tengen and 2. Tetris DS by Nintendo. 1 checks the two players' tetrominoes for collision against each other; 2 doesn't.
You can add TGM2, Shimizu Tetris, and one of the Spectrum Holobyte tetrises (Super Tetris?) to the list. All of these have an extra wide screen with cooperative play.
Tetris The Grand Master Ace has a cooperative 2-player mode as well. Player piece collisions are detected in TAP, TGMA and Shimizu Tetris.