Cab "A" lets say,which houses the mother board, would need to have the player 2 controls of cab "B" wired in parallel to its P2 controls- thats the easy part.
The rgb and sync outputs from the board would then have to be split in order to drive both monitors, this would require some buffering/amplification of these signals to run the two monitors (effectively half the impeadance of one cab) I will look for a simple buffer/amp circuit for this application.
Im sure there was a thread along these lines a while ago but have been unable to find it.
PS for safety reasons a good common ground between the two cabs would be a good idea.