I think I have the answer :-) Annoyingly simple - replace both bulbs. The man at the Ford dealer told me that both can go at the same time but it sounded so unlikely I ignored him, and I immediately thought it had to be something like a relay. Stupid me. I guess one bulb goes, and they share a wire or fuse so the voltage increases and the surge blows the second one which was probably on its way out. Fortunately I had a spare to confirm it. Thankfully it's not an expensive repair. There have been several other posts that say the same thing e.g. search for "Gone Out?, Where'd they go?" Now I have to put all those trim panels back together.