I play guitar (electric and acoustic) in a couple of bands. (Rock, Bluegrass, Folk, Twang)
I play mandolin (acoustic and electric) in a couple of bands. (Bluegrass, Swing Jazz, Jazz Standards, Rock, Twang)
I play bass (electric 4, 5, fretless 4 and upright in a couple of bands. (Bluegrass, Swing, Blues, Folk n Funk)
The fundamental thing for me is - approach each instrument as a seperate discipline.
Learn the proper technique of each instrument so I don't come off as looking like a frustrated guitarist playing bass and vice versa.
So far it's worked well for me as I am busy and gigging and having a blast.
Even make some $$
It's all music!