WHY YOU SHOULD REPLACE BOTH TORSION SPRINGS AT THE SAME TIME ?
If you have two torsion springs on your door, you should replace both of them. Most doors have springs with the same cycle life rating. In other words, when one spring breaks, the other spring will probably break before too much longer. Since you will be going to the trouble of changing one torsion spring, it is usually better to change your second spring as well.
Some doors, however, have two springs with different dimensions. Many times, the cycle life of the broken spring is shorter than the cycle life of the unbroken spring. This means that you may still have another couple thousand cycles left on your unbroken spring. If you only change one spring now, you will probably need to change your other spring fairly soon down the road. Therefore, we recommend that you still replace both springs, but that you purchase springs with the same length, inside diameter and wire size.
If this is the case, each of your new torsion springs will need to lift 1/2 of the total lift of your two old springs.