I’ve updated the downloads page (here) with Arduino code for the Game Master Controllers, as well as code for the PinScore Controllers and the Test Circuits.
The Game Master Controllers can be used as-is, or (more likely), as a template for creating your own games. Some are more complicated than others, but they’ve all been recently updated with a common, stable code. They should all have the capability to do multi-player (with the right build-switches set), and the “Miss Adventure” code is designed to to Multi-ball.
Each game has it’s own PinScore controller code. However, Jupiter Crush is the most advanced, so probably would make the best template.
There is also code to test switches, displays and solenoid drivers.
How to Build a Pinball Machine 