Not trying to be smart, but backup head for what reasons?
Is your band going on the road for the 1st time? Or are you playing local gigs?
Do you play shows with other bands or just your own band?
Just asking cause in 1000+ gigs over 20+ years and playing shows with many other bands, I never saw a bass amp fail except once and it was a guy who stopped at Guitar Center on the way to a wedding gig, bought a 1st gen Ampeg Class D amp, un-packed the amp at sound check and it didn't work. They happened to have rented my PA for the gig and I happened to have a Sansamp Bass DI in my bag which saved the bass player a long drive and very late start to the gig. We just ran bass through the PA and monitors. Not as good sounding as a D800, but none of the guests complained and everyone got paid.
As a bass player, on my 1st year as a full timer, I carried a spare head in the band van and never needed it. Just took up space in the van. Then I went through a phase of running 2 combo amps in stereo for 5 or so years. It was great, but I eventually got tired of hauling the extra rig. Still keep the spare amp,but it hasn't left the basement in years.
Odds are extremely good you'll never need a backup bass head for gigging especially if you play shows with other bands.
I still carry that old Sansamp Bass DI just in case...