How can I power one 12 volt outlet (feeding a XM radio transformer) from two separate 12 volt sources on a RV (ignition and coach power). Is it feasible to use 2 diodes?
The diodes should work, but be aware that they will drop ~0.7V or so, so you'll actually get ~11.3V instead of 12V. If the radio is okay with that, and if you can dissipate the heat caused by the diode drop * current drawn (may be small, but must be known), then you're okay. It won't backfeed either source.
If you don't want the voltage drop, then you can use a relay like this:
This has a disadvantage of dropping out the load momentarily if In2 turns on while In1 is already on, but it does avoid a diode drop.