I think if it's designed for 1 spring, that's all it needs. Once properly tensioned, it's all the help it needs. The motor in mine failed once and I could use one hand to raise/lower the door.

Be careful if you try to fix it yourself. That spring is under a lot of tension and really hurt you if it gets loose when tightening. Our spring broke a month or so ago (first time in about 15 years) and the guy actually clamped it with a wire clamp. It held for a couple of weeks while the new spring came in.