Are both PCB's powered up in one cab? If not, then just find the manuals for the PCB's and the pinouts, and make the Y cable to both.
I do suggest this,
Hook the power(5V) for the trackball up to the main power supply, not the board. Then put the X+X-/Y+Y- to the boards.
You don't have to power the trackball from the PCB. You can do that separately.
Do you understand?