While you might be able to make do with a single school backpack, there's always the risk that you'll be left in an awkward situation if a zipper breaks or a strap snaps. The trouble with having a backup backpack is that it might be deemed "uncool" by the time your child needs to use it. If you do buy a second backpack, it might be worth buying something fairly plain and timeless that isn't likely to go out of fashion within six months or so.