Garage door springs should be replaced as a set. The reason is once one spring breaks, the other one is probably not far behind, especially considering how springs weaken over time. Combining a new spring with an older one will likely cause an imbalance in your garage door's operation. If you own 2 separate garage doors and only one spring has broken, it's strongly recommended to replace the springs on each door if their springs are around the same age. Depending on your door's position, when a spring breaks it can cause serious injury as well as damage your opener.