what is the likely cause of a single fluorescent tube not working in a light fixture with a ballast that supports two tubes?
I buy most of my ballasts on ebay for a couple of bucks, they are new pulls (seller bought new fixtures and immediately converted to LED). I combine them with the newest 90+ CRI tubes ($2 each) and the light is amazing! Don't look at the fixtures, look at the stuff the fixture is lighting up.  

ALL fixtures can take electronic ballasts, I have converted some 1940 era fixtures (after giving them a trip through the paint shop). Those old fixtures are well built.  The only exceptions are very cheap fixtures with weird unobtanium tombstones or no space to fit a ballast.

If the ballast is instant start, it is possible for an electronics failure to take out one side.  But it's more likely a tombstone problem (or wiring).  A bag of spare tombstones (60 cents ea. at 1000bulbs) is essential even if converting to LED.  

I dislike instant start ballasts and prefer rapid or programmed start when obtainable at sane price.  You want programmed start for hard to reach locations or frequently cycled lights e.g. Bathroom or on a motion sensor.  

Tubes don't "burn out", they get dimmer, scorch their ends and then flicker or barely light.  An abrupt failure is tombstone or ballast.