Assuming you've got two senders, one in each tank, and no "equalizer" connection between them to keep the fuel level even in each (which would only need one sender). All you need is a switch that has two "on" positions (up/down, left/right, however you want to orient). The switch completes the circuit, so one sender to one side of the switch, the other to the other. Then to the guage, it's only going to read the circuit that's "live".

Don, with your ground clearance issues you may want to reconsider. You could mount a verticle tank behind the seat and route the filler to the tulip panel (personally I don't care for the filler in the trunk thing). Using a flush filler door it would hardly be visible. Remember, if the front or rear wheel drops into a low spot the clearance at the center of the car is reduced, so I don't think you're going to have space to spare.