I always try and have two on hand that are 'interchangeable' to an extent. I'm currently using a 5 and a 4 string (tuned BEAD) that are the same bass model, same pickups, woods, etc. The idea for me is to grab the backup in case of something broken (string, jack, strap, etc) and jump right back into the song or set. I would suggest switching at rehearsal between songs and see how quick you can make the necessary amp adjustments. To me, if you can't do it on the fly, then having a true backup makes little sense.
