On Sat, 08 Dec 2012 05:10:09 -0600, Vic Smith wrote:
2. When one spring breaks, the door goes up softer too!
The danger and effort is cut in half with a 2 spring.

I totally agree that your two points above are 100% correct.

When 1-of-2 torsion springs break:
1. The door going down is (much) safer!
2. Lifting the door up is (much) easier!

But, there's another not-so-obvious advantage of having two springs
on a double-car door when spring breaks while the door is moving up.
3. When one spring breaks, the door goes up softer too!

It's difficult to explain, so allow me simply quote Dan Musick himself:
How to Convert from One Garage Door Spring to Two
http://ddmgaragedoors.com/diy-instru...conversion.php

[ verbatim ]
Other problems frequently ensue when a single spring is used on a double-car
garage door. Many manufacturers have cut costs by using a single spring on a
double-wide 16' steel garage door. If the spring breaks near the stationary
cone, a large portion of the spring spins loose with the winding cone secured
to the shaft. This causes a strong force to pull on the cables, often leading
to one or two broken cables. If the opener up force is set too strong, it is
more likely to wreck the top section if not the whole door. A second spring
helps to keep the tension when the first spring breaks, resulting in fewer
broken cables and less damage to the garage door itself.
[ / verbatim ]