In fact, you can also have a different touch input devices per seat! I'm not aware of actual hardware that would make this feasible, but if you invent a touchs

In fact, you can also have a different touch input devices per seat! I'm not aware of actual hardware that would make this feasible, but if you invent a touchscreen which recognises between two people's different fingers, you could have a client which tracks who clicks where without any protocol changes.