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I can't really see how opening all the blades at once would necessarily put any unusual strain on a knife.
Once a blade is fully opened the spring returns to its original position and the strain would be minimal. If a blade is opened to its half-stop position, again, the spring is returned to its original position and the strain would be minimal. The only position in which the spring is stressed is at the points between closed, half-stop, and open, and therefore, I could see that you would not want to open two blades (on the same spring) to any position stressing the spring at the same time.
If you open a knife like a pen knife to a position such as half open (on a knife not having half-stops) you would stress the backspring to its maximum stress and, if the spring were to break, I would think that that would be the point at which it would do so.
JMO
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