Multiplayer would indeed be a nice idea. The nice thing is that you don't have to play an entire ball and still score points. You could consider scoring depending on correct inputs per time played. Look forward to this.
a mutliplayer version is high on our list and have had many discussions about this with our development teams and supporters. The question right now is do we make the multiplayer version for Java or directly for a Windows Application or for others.
Multiplayer can be fun and we could provide a simplified version of the ball to enable "speed" playing, time playing or the first person who has one sudoku completed on the ball. We have a long, very long, list of ideas on how we could make the multiplayer really fun. Multiplayer also doesn't need to be restricted to 2 users but could also have 3 or more users. Java performance might be a problem hence a PC version or other version is being looked.