Both cables get replaced as a pair, not one at a time, even if only one has broken. The other one is the same age and under the same load - fixing one side and leaving the other is a callback waiting to happen. We check the drums for wear grooves, check the bottom brackets for stress cracks, and check the track alignment, since a cable failure on an older door often means the door has been running slightly out of true for a while before the cable gave out.
On a door with original torsion hardware from this build period, we also check the spring itself while we're in there - not to upsell a spring you don't need, but because a spring near the end of its life and a stretched cable tend to show up around the same age. If the spring's fine, we leave it. If it's close to done, we tell you and let you decide.