Power, video, inputs, and outputs -- you need to switch them all, except for ground.
You could use a parallel port A-B switch to switch the input and output lines, and maybe the video. (depends on the switch specs -- may require a relay instead)
For the 5v and 12v power, you can use a 5v DPDT solid state relay with the power supply connected to COM, PCB1 connected to NC, and PCB2 connected to NO.
One pole handles 5v, the other handles 12v.
When the parallel port switch is set to A, the I/O lines are connected to PCB1 and no operating power is applied to the relay so PCB power flows from the power supply to the relay COM terminal to the relay NC terminal to PCB1.
When the parallel port switch is set to B, the I/O lines are connected to PCB2 and operating power is applied to the relay via one of the parallel port wires so PCB power flows from the power supply to the relay COM terminal to the relay NO terminal to PCB2.
Scott