Double your probabilities with a fly fishing dropper rig
One of the best, but often overlooked, ways to succeed in fishing is to double your listings. For fly throwers, this can be done by using a fly fishing dropper rig, which is particularly useful when fishing for trout and pan fish, as well as when casting small, streamlined flies.
A fly fishing dropper rig is simply a combination of two flies with one fly on one leader above the other. This is achieved by attaching an auxiliary fly to its own short tippet on a leader in front of the main fly. Anglers call this auxiliary fly a "drip fly", which is not a type of fly (like dry, wet or streamer flies), but just a term that describes how it is placed on the leader.
Drip flies can be used for nymph, wet fly and streamer fishing as well as in combination with dry flies. They are easiest to use when tying your own knotted tapered leader by allowing a few inches of overlap to hang from the knot used to join different thicknesses of leader material. Tie a drip fly to the protruding line and the primary fly to the end of the tip.
For example, a fly fishing guide consists of a main length of guide line with a section of the lighter line called a tippet at the end. The top and main trunk of the leader are connected with a blood or university knot. As you tie the knot, let about a 12 inch strand dangle on the lighter line, then attach your auxiliary fly to the end. Leaves a drip fly approximately 6 to 8 inches from the main body of the leader. Finish the fly fishing dropper rig by attaching your primary fly to the end of the main tip.
(Note: it is possible to add a dropper fly with a dropper loop knot, although this is not my preference as this knot is better for heavier fishing and bottom fishing applications.)
Which one flies?
While two different patterns of the same species of fly can be fished at the same time, such as two wet flies or two streamers, it is common to use a nymph on the pipette in conjunction with a dry one as the main fly. In some cases, the dry fly acts as a strike indicator for the nymph.
Using such a fly fishing tackle is often a good trick for prospecting when no insect activity is visible or before a hatch appears. When two fish hit the dry fly in a row, it is easy to cut off the nymph. By the way, a drip fly can also be fished with a bait, especially with a small spoon or spinner.
Another two-fly option
You can fish two flies in other ways than a dropper fly rig. Technically speaking, these are not drippers as the auxiliary fly is fished directly behind the main fly instead of in front of it. This is more of a tandem arrangement and is achieved via a short leader attached to the eye of the main fly hook or to the bend of the main fly hook.
Tying to the eye is generally better for nymphs, while tying to the bend is better for streamers, provided the main hook has a definite barb. These placements have the advantage of being less tangled (during a throw) than a dropper setup.