I don't carry a spare headlight, though perhaps I should. On the bike I normally commute on, I have a dynamo powered B&M headlight, a dynamo powered tail light, a USB rechargeable flashing tail light, and a button battery powered spoke light. The headlight has proven to be completely reliable. Any of the other lights can fail without making me feel nervous, though that's rare. The rechargeable tail light doesn't need frequent recharging. I don't know how long it takes to discharge, because I charge it once every two or three weeks, and that's shorter than the discharge cycle. The button battery lasts a few months before needing replacement. The spoke light is not very bright but because of its placement, it is very noticeable.

Occasionally, I ride a racing bike to work. I like to keep that bike light and don't bolt anything to it. I use a USB rechargeable Cygolite headlight which I live in (irrational) fear that it will discharge. I use the same rechargeable tail light and button battery spoke light, which means I have only one rear-facing tail light instead of two.