If you can be a little more patient (okay-- maybe a *lot* more patient!), I have an idea or two up my sleeve that might help, but I doubt I could get anything to you before next week.
Okay, you twisted my arm. (Well, I twisted my own arm; so sue me.) I noticed that you aren't using any of the players, missiles, or the ball in your kernel (aside from the score)-- at least, you weren't in the earlier versions I looked at. So I'm going to create a custom kernel for you that will let you draw one or two standard-size Tetris playing fields-- 10 blocks across, and 20 rows tall-- without needing to use the Superchip. And I'm going to have the kernel check each row as it's drawn to see if it's all filled, and set a bit flag for that row if it is-- so that all you have to do to see whether or not a given row is filled is check its bit flag after executing the drawscreen command.
By the way, I suggest that you use the "score color mode" for the two-player game, so the left side of the screen can be drawn in one color, and the right side of the screen can be drawn in another color-- e.g., blue for the left player and green for the right player, or yellow and red, or red and blue, or whatever two colors the players want to pick.