I'm probably already too late, but...
it depends how the cabinets are wired up, how the monitor video grounds are wired up, if the monitors are isolated, and if the power supply for the game is isolated from mains or not...possibly even how the plug is wired up. (possible neutral/hot reversal)
too many unknowns with regards to hotwiring 2 cabinets together for testing.
it's far easier to just pull a working board and dropping it in the cab to test.
generally if i'm testing an "isolation required" monitor, i'll isolate the power supply for the game as well just in case. (I have a special power plug on the test bench that's isolated with the monitor power.)