I never carry a backup on my person. If I'm going on a trip, I usually have at least two, mostly so I can carry whichever is most convenient for a given day's itinerary. Depending on where I'm going and if I'm camping out, I might also have a long gun or two.
With my lifestyle, travel habits, situational awareness and general cautiousness, I figure that there's virtually zero chance that I'll need the gun I'm carrying. The odds of needing it AND a backup are astronomical.