How to turn on gauge lights from multiple sources
It sounds as if two diodes will do the trick.  You will want to purchase diodes that are large enough - 1N5402 diodes are good for 3 Amps and should work nicely for gauge lamps.

Each diode has a band at one end.  Connect the ends of the diodes with the bands together and to your gauge lights.  The free end of each diode goes to one of the lighting circuits that you want to take power from.

Do note that this will introduce a slight voltage drop to the gauge lamps (about 0.8V @ 1 Amp) but I don't think that you will notice the drop in brightness.

If it turns out that you can't afford any voltage drop at all, come back and modify your original question to say that.  You can do the same thing using a SPDT relay with 12V coil but it costs more than these two diodes.