EDIT: i guess i should have read the previous post, since it lays out the same thing i am suggesting below... oh well:
there is a way to do this, however not in the traditional sense. it also has a few hardware requirements that you may or may not meet.
you can set up the second cabinet as a remote terminal for cabinet one, using a KVM switch. just to give you an idea how this will work, I am going to make a few assumptions about your cabinets...it may or may not work given your current hardware configuration:
Cabinet 1 would require that the video card has two outputs and is capable of cloning the display (most cards do this)
cabinet 1 has a controller interface that connects via USB
the monitors in cabinet 2 needs to be capable of runnign at the same resolution that cabinet 1 is using.
cabinet 1 is set up like normal, player 1's controlls are using mame defaults for player 1.
for the second cabinet, you have a controller interface that is PS2, such as an Ipac. you wire player 1's controls to the player 2 side of the encoder, and then configure mame to use those keys for player 1 instead. you connect the encoder in cabinet 2 to the keyboard input on a KVM switch. you connect cabinet 2's monitor to the KVM switch also. you then run one set of cables from the KVM to each PC.
that way, when you are using cabinet 2, it works just like normal, then you can press a button on the kvm, and it isntantly displays whatever is on cabinet 1. this also connects the ipac on cabinet 2, to teh ps2 port on cabinet 1, and since that ipac is already wired up as player 2, you will be able to use it as the player 2 controls.
player 1 will be on cabinet 1, and player 2 will be on cabinet 2....when you are done, just press the kvm button and cabinet 2 is back to it's normal operation and you have two independent arcade cabinets once again.
i dont know if this will work in your situation..and i'm sure you could tweak a few things here and there to adapt this plan, but i just wanted to let you know that it is possible.
good luck