My main gigging bass right now is my stack knob '62 USA Jazz Bass, and the usual backup is my 1974 Fender Jazz bass (actually the actual very first bass guitar I ever played. I inherited it after the owner, my friend Jimmy Davidson, passed away in February 2010).
Unfortunately, I can't use it on my own gigs right now as my friend Paul Turner has it at his place.
So, my 'other' bass on gigs right now is a '62 reissue USA Fender P-Bass.
Saying that, I'm sure most of us would agree that we use only a single instrument for the vast majority of gigs. I've been playing bass for over 30 years and have never had any of my basses fault on me during a gig. I've had a few amp hiccups, but never bass ones.