You don't have to replace the pair, they are not made in matching pairs! they are made to a specification that will be well within the tolerances of the manufacturer, so not crucial like replacing shock absorbers for example...
Coil springs snap, because of a simple manufacturing fault, when they are powder coated, they are hung up, a tiny area is missed by the coating in this process, and remains in bare metal, that of course starts to rust, given a few years and some speed humps, the week point snaps
Thom