Could it be that more jacks were made because they are easier to make ?
Jim
You could have a point here, single springs are trickier to make if you're particular about blade-rub for sure.
Did a review of some small knives on my desk as I write: GEC Conductor, CASE Pen (yellow) RR Mini Copperhead, Buck 309 (yellow smooth delrin) CASE Crandall P.Nut, Queen Cutlery No.2 Jack. The GEC is by far the best made, no blade rub or play and gives fine grip in the hand with its swell centre. Its blade is the longest and broadest of the knives, very impressive blade in a small frame. It also costs a lot more than the others. CASE Pen is not far behind in ergos but the build is more rudimentary, blade not as broad as the GEC and the blades droop down on open. Fine to handle though. The RR has its qualities too, bit smaller than the other two but nicely made, good in hand and the Copperhead design ensures no rips in the pockets (note the GEC tangs are fearsomely sharp and needed smoothing down with a stone).
So to the double springs or "two thick". The Buck has appeal, well made, all stainless construction, good quality blade steel. But....very narrow blades indeed that I can't see the point in being sabre ground. Hard to grip this shiny yellow delrin securely plus it's a very narrow handle too. It is a lot better in hand when in sawn black delrin, this is grippy but it's too small for my taste and I have small hands too. Good back-up PSK type of emergency knife though, it cuts well. The CASE Crandall has beautiful thick bone scales and tangs that are low, so they're easy on the pocket. The pen blade is very hard to open and the P.Nut is the only knife here with half-stops, come to feel that I prefer the smoothness of a small knife WITHOUT half-stops though. Yet I prefer the CASE Pen as it's longer, so I much prefer it in the hand. Yes, I know I'm in a minority here but that's how it is for me... Lastly, a better Jack than the P.Nut? Yes I think so, the Queen Serp Jack No.2 is about Conductor size (handle narrower) and offers very good comfortable grip, decent Clip blade still not as broad as the Conductor though. It is heavier, perhaps D2 is denser?? Could just be from having thicker springs too. It offers nail-nicks on both sides, a rarity these days.
Still find the single-spring Conductor to be superior (build, size, weight, blade size,grip) but it's close. If CASE were to offer the Pen in some bone scales (and re-tool a bit for better fit) then this would also be very tempting indeed. All these knives have undeniable virtues though as they are different from each other.
Well, I find the Queen to be better than the Nut. But, it's all too obvious to see the appeal of the Nut and the creation of the notorious Cult of the Peanut. Somehow The Cult of the No.2 just doesn't seem to have the same allure....

Thanks, Will