Backup instruments didn't really become a thing until the band had completed their first album and was supposed to be(come) a professional band. During the club days in 1973 they each had one guitar, period. But when they (hopefully) had to play several nights a week in far off places it was paramount that the show would go on. Instrument malfunctions, should they happen, would not be allowed to derail the performances. Thus Gene ended up with a Fender Precision which can sometimes be seen standing on his side of the stage if the stage was large enough and photographers shot a certain angle.
This bass first shows up during the rehearsals for the New Years show at Academy of Music and when they play their first showcase as a Casablanca recording artist at Filmore East on January 8, 1974, it can be seen standing on Gene's side of the stage.