I think the Coin Unit should advance one step for each player as they're coined up so if a two player game doesn't work, I'd start there.
Target Alpha 2 player game (resized).jpg
In the schematic above the four parallel paths at the bottom are used to advance the Player Unit when the ball drains based on how many players there are. All games use the bottom path with the Motor 2C switch. Multiplayer games also use one of three paths just above it through the Coin Unit. I think two player games use the path in red. If two player games aren't working correctly I'd look at the Coin Unit and the P2G switch on the Player Unit.