Then what are you saying? Leon Wilkeson left the band before recording the first album; guitarist Ed King was brought in to cover bass duties. Leon then had a change of heart, rejoined, with King going back to guitar, adding to Rossington and Collins for triple attack they were famous for, but no dual basses.
I'll do some investigation. I have a member of the extended family who did something like 30+ shows of theirs in the early days and mentioned it. The way they described it, it was more like 'here is our friend [insert name] he's hanging with us tonight.' Don't know, wasn't there. Just know a lot of bands do a lot of weird crap when they are bored on tour so I absolutely believe they had a local friend they'd let hang some nights.