There's no any "Start button for pool game" - just a button named "Start" on controller, the button you press to pause the game. If you press it on your second pad while being in pool set-up screen - you'll play 2 player pool game.
SAAC is a very old and buggy third-party software. GInput is a great plugin that re-enables native dual controllers support and doesn't have bugs. So yes - I suggest you to use GInput instead of SAAC.
That was posted in the first message in DEPENDENCES section
I got it, I tried to use GInput for Bluetooth Gamepad (Not Xbox or Playstation Joystick) but it won't work.