thats a pretty cool idea also. two cabinets. to top it off, with some creative wiring and button mapping, you could use that as two independent cabinets, or connect them so that players 1 and 2 are on one cabinet, and players 3 and 4 on the second one. it wouldnt even be that difficult. the main cabinet would be set up like normal, it would just need dual monitor outputs and configured to show the same image on both monitors. (i believe most cards are capable of this, i know my radeon 9600xt is) and preferably a usb encoder.
the second cabinet you would reconfigure the encoder so that it uses the standard keymappings of players 3 and 4, then reconfigure mame to use those buttons for players 1 and 2. that way it would function as a normal cabinet. assuming that your encoder uses a ps2 port, all you would need is a 2 port kvm switch. you connect one set of cables to the PC inside the cabinet, and you run the other set to the other cabinet.
both cabinets would function as normal 2 player cabinets, but all you would have to do is hit the KVM button on cabinet 2, and it would be displaying whatever is on cabinet 1, plus the control panel would now be connected to cabinet 1, and already configured as players 3 and 4. so you have instant 4 player TMNT action, with no crowding or standing at stupid angles. the only real down side is that the cabinets would have to be relativly close to each other, and you would have to use VGA compatible monitors (i havent seen any video cards that have dual outputs for a TV).
sorry to go off on a tangent there, but it is a feasable option.
anyway, good luck with whatever you try to build, and please post lots of pics of your progress.