Anchor Fly?

[DUB]

If you want both or all (Colorado allows 3 flies) patterns to be at the same depth (moving water), make the anchor fly the closet to your leader. If you want one or more flies to ride higher in the water column, make the anchor fly the farthest from your leader. Both techniques work in different situations.

[Byron haugh]

By placing the heavier or anchor fly first, with the lighter or unweighted fly off the bend, it allows the dropper fly to drift more freely. Its drift will vary in depth as it travels dependant on the length of the dropper, often rising above the anchor.

Although I don't fish with two subsurface flies, this is what I have always understood to be the method and the reasoning behind it.

[DUB]

Buoyancy of the trailing fly, length of tippet between dropper and anchor, current speed, and conflicting currents all have an impact on where the dropper will ride in the water column. Generally it is just a few inches above the anchor if any.There are times when that is the perfect location for the dropper.