Why a SST on a double gun?

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Kenny, according to W.W. Greener (The Gun and Its Development), the single trigger on a "double-barrelled" gun appeared at least as early as the late 17th century. Greener stated "the single trigger undoubtedly possesses advantages over the two-trigger gun, principally the facility with which two barrels can be fired in rapid succession; the same length of stock is secured (LOP)-with most mechanisms- for both barrels, and there is no necessity to relax the grip of the stock. What these advantages mean to the sportsman can only be fully appreciated by a fair trial at game."

For gun manufacturers the choice was simple, the competition had them, people wanted them and were willing to pay.