As for what do I do when the GPS fails... I use the other one. If that fails too I get out the handheld. At this point I've presumably lost ship's power anyway so the ADF wouldn't help me.
Glib, but rather assumes a problem with the individual unit(s), not something systematically affecting the whole aeroplane/ geographical area (seem to remember a UK NOTAM about that occurring), or, heaven forbid, a problem with the GPS net as a whole.
Fairly unlikely I grant you.. but all the eggs in one basket was never a great plan IMHO; to have the tools in the aeroplane and not know how to use them is just negligent
