You only have to replace what's not working properly but any conscientious mechanic would replace both to insure even braking. In return for being conscientious and having your best interest at heart he will likely be accused of selling more parts than are needed, but he knows that whatever happened to one caliper is likely to happen to the other one since it's just as old and lives in the same environmental conditions. "Seized" can mean the piston is sticking in the caliper and not applying and retracting properly or the caliper can be stuck on its mount due to rust and it's not self-adjusting properly. Either way it is likely to happen to the other caliper soon too.