I got one of those (a Green head .15) back in the late '50's by selling greeting cards. Part of a package with a Berkeley Dauntless. I never set it up with both needles, but the idea was that with flow through one needle the engine would run normally fast. Then there was a "switch" that would with run fuel through both needles or through the second one that was set very rich. Hence the engine would slow down. I never had the "switch" but read about the system in a magazine or the included instructions. Way too long ago to remember any details. Surely it did not work well.
Chuck