I gig a lot, but don't have any bass that us purely a 'backup'. I generally take two basses to a gig, but i'll usually only use one. I use different basses for different bands, and one will often just act as a backup for the main bass on another gig. There's an assumption that I won't use the 'backup', but if I do, it will be up to my standards as it's the main bass for another project.
If I just had one project, with no interest for different musical situations, then i'd probably just buy two of the same bass.....or perhaps something wildly different...who knows haha.
Si