Feedback Tetris Clone Co-Op Concept

I have been working on a Tetris clone with pieces that consist of 1, 2, and 3 segments. I basically have everything I have planned. But as for the Co-Op mode that I want to add? I have no idea how to approach this. Basically, you and a partner literally share the same grid and work together to score points. But the concepts are scrambling inside of my head, and I need some help making a decision. Here are the concepts I have in mind for the grid:

• 9x8 Grid w/Shared Lane
• 9x8 Shared Grid
• 10x8 Grid w/Shared Lane
• 10x8 Separate Sides
• 10x8 Shared Grid

Small grids? Yes. Reason? Like I said before, the pieces consist of 1, 2, and 3 segments. To summarize, I'm making a game that's not only simple to play but challenging to master in terms of making a high score. Tell me in your personal opinion what you think works, and why. Any help is appreciated.

 

To keep it simple I would go separate sides.
Each player has own side, but to cancel lines, both sides need to have line complete.

This way players need to work together, to complete same lines.