On a torsion system, we take tension off the spring first, then pull both cables - even if only one broke, the other side has been carrying uneven load and is usually close behind. We match the new cable to the drop and drum size on the door, since a cable that's even a little short or long throws the door out of level.
On houses from Lucas's building era, roughly two decades in, the drums and bottom brackets have usually taken 20-plus years of coil-and-uncoil cycles. We check those parts while we're in there, because a fresh cable on a worn drum groove will just fray again. If the bracket's rusted or the drum's grooved from wear, that gets flagged before we close the job out - a cable-only fix on bad hardware isn't a fix, it's a callback waiting to happen.
We don't carry every cable length and drum size on the truck for every door style. If your door's an odd or older configuration, we may need to confirm the part before the visit is finished rather than promise it on the spot.