Generally guitar players can also play bass, but due to the size of the strings it can be difficult to play and may require a few hours adjustment to get used to the fretboard and general size of the strings, higher up the fret of the bass it is very different to a guitar and bending is much more difficult to do due to the tension and size of the strings. Also going the other way bass players tend to be musicians who have picked up both instruments so it’s probably just that they have learned both stringed instruments, especially when writing songs because it would be easy for a musician writing a song to play the bass track and guitar track when recording songs in the studio.