Think about it. If one is good, two is better. Trout feed under the surface anywhere up to 90 percent of the time, according to conventional wisdom. Subsurface flies meet the feeding expectations and the behavior patterns of trout and their food. When trout feed on the insects floating on the surface of rivers or lakes, they rarely do so to the exclusion of underwater goodies.
Trout think subsurface first, surface second.
Prospecting for trout By adding a second fly, the angler gains greater coverage. Even if the trout are keying on insects on top of the water, two dry flies or a dry fly with a subsurface dropper covers more water, more lies, more trout. If you fish with more than one fly, you are fishing multiple flies. These can be two ore more nymphs, two or more wet flies, a large streamer and a tailing nymph, a dropper rig with a dry fly followed by a nymph, or two dry flies. When you fish a dry fly with a subsurface fly below it, you appeal to the trout's interest above and below the surface.