Think about it. If one is good, two is better. And the creatures that live in the water, the food that trout eat, mayflies and caddis and stoneflies and baitfish and crayfish and mysis shrimp live in the water. Subsurface flies meet the feeding expectations and the behavior patterns of trout and their food. When trout feed on insects on top of the water, they rarely do so to the exclusion of subsurface goodies.
Trout think subsurface first, surface second.
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. 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 are appealing to the trout's interest above and below the surface.