Why have one Air Force One when you could have two? The plane technically isn't even called 'Air Force One' - that's just its call sign when it's flying. The call sign can apply to one of two almost-identical jets that the President uses.
It would be embarrassing if the plane broke down and the President had to make alternative arrangements, so there are always a minimum of two planes in service which can perform 'Air Force One' duties whenever required.