I ALWAYS bring 2 basses to a paid gig. I think it is the professional thing to do as things can go wrong, but that is only IMO. If you only bring one bass, good on ya.
I bought a Levy's double bass gig bag and it is basically the same as carrying a single guitar case weight wise. I may never play the 2nd bass but it is there if I break a string (super rare), battery dies, a solder breaks, a bridge screw comes loose, a nut breaks, etc.
Best $200 and change I have spent on a case/gig bag and it goes everywhere with me.
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Well you raise a very good point here if it's a paid gig, especially a sub or hired gun situation. Only problem I found with double bags is that they can get heavy. Plus I'm already invested x2 in Undecover Bags. I can put everything I need for a gig except my cab in mine!
I am also using practicing with whatever bass I bring to a gig just before I head off for the gig so can usually spot any problems then. Realize this isn't possible for everyone in all cases.

So the past several years I have been taking two, a p-bass and a jazz. I've still never had a problem where a second bass was needed.