A mechanical example would be passing the ball between players, but that doesn't mean that it would work in this case. A perfect mechanic would be very simple and allow players to play, interact and express themselves for fun. In Magicka you do that by simply spell casting as usual but things can go in wildly different directions when multiple players are involved. In Killing Floor you can throw cash to each other to share and for fun.
It should be some form of assist as right now unless player is forced to rely on another through walls, it will either be one player getting most of the holes due to skill difference or more skilled player having to wait for less skilled one due to level design.
You might want to make the game more chaotic too by enabling more scenarios. Angle snapping is cool, but all that's really needed is a more generous hole size.
Tower defense might be too much, but imagine getting a bomb (by accident or not) and having to quickly hit it away before it automatically explodes, maybe losing a turn as a result, making an additional obstacle crater for yourself or sabotaging the other player.