I use a double set up when I am trying to pin point what the fish are taking. I use a larger b.h. 24-32 inches on the tippet then 18-20 inches below the first fly as a dropper. It takes some getting use to but I ain't usually smart enough to figure out what the fish are taking so it is sorta like a buffet for the fish.
I also fish droppers off a dry and the air knot as Hugh H. told me is difficult to master but useful and increases your changes of hooking up. I also use a dropper to increase drag, sometimes they just want it slowed down.
I hardly ever use only one fly unless I can zone in on what they taking.