Build two independent circuits with one switch
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Using a single-pole normally-open switch to select '5 V' or 12 V supply.

SW1 open: 5 V is supplied to the load via D1. A silicon diode will drop 0.7 V. A Schottky diode will drop about 0.3 V.
SW1 closed: 12 V is supplied to the load. D1 prevents backfeed into the 5 V power supply.





simulate this circuit
Figure 2. Switch to disconnect two loads.

SW1 will disconnect the common return from both loads to the common ground at the power supplies.
D1 prevents V1 back-feeding into V2 when SW1 is open. It will cause a voltage drop in normal operation as described above.

This isn't an elegant circuit. Switching negatives in a negative ground circuit can lead to all sorts of confusion. It is, however, common on automobile circuits.