Torsion systems are the primary systems used to operate garage doors, torsion spring mounted on an torsion tube that generating the torque that necessary for lifting and carrying the door..
With one torsion spring if its break there is no torque left to carry the full mass of the garage door however if two or more springs are installed then this issue can be avoided.
With just one torsion spring there is no pressure at all on the door so the door cant move and that can make damage for the cables, pulleys, hinges and rollers and resulting an off track door. Having two springs installed helps alleviate these issues by providing balance and pressure from the other spring.
In conclusion it is highly recommended that two torsion springs be installed in order to ensure safety when lifting your garage door along with reducing any potential damage.