Yes, you have to replace both springs. The springs on your garage door are similar to those on a car. Both are torsion springs: the type of spring with a lot of bend and twist. If you replace just one of them, the new spring will bind because the other will do more work for the same tension, resulting in a weak spring. If you replace just one, you'll find out the hard way; within a few months, I'm back replacing the other spring when it snaps. If you replace both, your door will operate correctly, with less stress on the rest of your door mechanisms.