If anyone is interested in doing this here is the lowdown.
Both AV boards have pre-exisiting sockets for the communications chips. Populate them with a MAX 239 RS232 , and a 16C350 UART.

You also need the latest Game Eprom v3.1 and a new U2 sound eprom v3.0 installed.

Now you need to link the pins via the brown connector next to the two installed comms chips.

Make a cable of suitable length. You only need two conductors and a ground screen. Connect the ground screen to pin 1, and the data wires to pin 2 and 3, but swap pins 2 and 3 around on opposite ends of the cable. This cable can be routed via the ventilation holes in the top of the cabinet by reoving one of the fixing screws.

Now for the bit thats not so obvious.

On each machine , go to feature adjustments, and set first machine to Player 1, and second machine to Player 2.

Then go to Utilities, Pre-Sets and Select "Install Serial Link 1" on machine one. And "Install Serial Link 2" on machine 2.

If everything has gone to plan, pressing either start button will initiate comms link, wait for a prompt on the second machine, then press the second start button. Now you're playing head to head, and all sounds are synchronised.