that sounds like the link settings are incorrect or not everything is hooked up
You didn't buy a "pair of cabs" you bought a "twin unit" it's a single machine with 2 seats. you might notice that you only have 1 coin box and each side is missing half of the plastic cladding. The default link configuration requires that the two boards are connected and powered up together. Virtual On always shipped this way, even in candy cab form it shipped in a versus unit.
There are 2 optical tosslink cables running to each Model 2 board, and they pass through an adapter in the single shared coin box of the two side of the machine.
you want to make sure TX on one board is routed to RX on the other board, and vise-versa
those need to be hooked up and both boards powered up.
you can reconfigure the PCB for single unit play but if you've got both halves of the twin unit then I'd recommend just running it as it was designed.