I often fish a dry with a similar emerger below or a dry or emerger with a nymph below. I do that if I have a good idea about what the trout are feeding on. If I am not sure I will fish different types of insects to see what it is they are after. I also use a bead head wooly booger with a nymph either above or below to get it down deep. I like to experiment a lot. It adds to the entertainment value of the days fishing and increases my knowledge base.