” is there a reason he might use one guitar in the studio and another in concert?”
There’s probably a reason or two, but not of the earth-shaking variety. Perhaps one plays a litte better, one stays in tune a little better, one is in a (heavy) anvil-style road case near the door, road-ready, and the other is a back room where he has to get off his ass and go get it.
In concert there are always extra hired guitar players.
?? Extra guitars, as backups, but guitar players?
Yes. Always a base player after Roger Waters left, usually another guitar player, sometimes an extra keyboard player, and at least once a second drummer (Pulse at Earl's Court?).