When one of a pair of torsion springs breaks, the other is the same age and has done the same number of cycles. It is not a question of whether it will go, but when.
The arithmetic
The second spring on the same visit costs a fraction of a second call-out. If it breaks in a month you pay the labour twice.
It also protects the door
Mismatched springs pull unevenly, which twists the door in the tracks and wears rollers on one side.