Many anglers have considerable difficulty in selecting which fly to use. Should it be a dry, an emerger, or nymph? They may look at the stream and see an occasional rise. Obviously fish are coming to the surface but it might be totally random. Now what! My suggestion is to use more than one fly or a tandem rig. I've met anglers that will use a brace of flies (more than two) to cover all parts of the water column. This is a very common practice in Europe, which can lay claim to some of the best fishermen in the world.

A few may find the task of tying on multiple flies time consuming but it's well worth the effort. By fishing a multiple fly setup, you can locate and key in on what the fish are feeding on in half the time.

The most common method is using a Dry/Dropper rig. This highly effective rig provides the opportunity for the angler to dabble with nymph fishing while still focusing on the dry fly. Dry/Dropper rigs most often exist as a dry fly with a weighted or beadhead nymph tied 12" - 24" below it, depending on the depth of the water being fished. The technique allows the angler to anticipate the surface bite, while also prospecting all of the water below. When fishing a dry fly with a subsurface fly below it, it appeals to the trout's interest above and below the surface and imitates two different stages of insect life. A favorite benefit of using this type of setup is using the dry fly as a strike indicator. This is a highly effective and simple technique. Keep in mind that a good dry fly floatant will need to be applied to the top fly more frequently to compensate for the added weight of the dropper (usually a beadhead).

A double dry fly setup can be very productive on days when trout are keying in on the surface. When this occurs, adding another dry fly is an efficient way for the angler to figure out what pattern the trout are taking. A good use of this setup is when fishing a small and barely visible pattern such as a midge or Griffith's Gnat. Although the smaller dry can still not be seen, it gives the angler a close proximity by just keying in on the primary top fly. Any surface action caused by a trout within 24" is usually a good indication that the trailing fly was just struck at. This gives the trout a choice and the fisherman a better chance.

In deeper and faster moving waters fishing heavy double nymph rigs may be the most effective technique. The most important factor when using this type of rig is to use a splitshot heavy enough to keep the flies down in front of the fish and moving at the same speed as the current. This rig is effective in fishing deep runs and swift water.
I recently heard a good piece of advice from Jack Dennis. When fishing tandem rigs it's usually a good idea to fish different stages of the same insect if you are reasonably certain about the hatch that is or will be taking place.

I've got one last comment. When prospecting for trout in the summer months always try the Dry/Dropper first. With this, you will cover more water and more lies. The dry fly can be anything from a fat-bodied terrestrial to natural you notice or suspect to be there. The dropper fly should be tied to either the hook bend or the eye of the dry fly and should resemble something that is available in the water you are fishing.