Yes, you should replace both springs at the same time. If you have a torsion spring break, it is likely that the other spring will soon break as well. Replacing both at once ensures that the door will be safe and secure for many years to come! You can replace one spring at a time if you'd like, but it may cost more than replacing both at once because of labor costs involved in removing and installing each individual torsion spring or increased shipping costs if you do it yourself.