John Rhoades wrote the following:

"Where I have more difficulty is getting hackle barbule tips even, for example when I mix grizzly and brown hackle barbules together for the tails of an Adams. However, even though I try to get them even, and like the looks of my flies tied with evenly paired hackle barbules, I doubt that it matters one way or the other to the fish."

So here's a tip I saw on a video. I think it was a video by Charlie Cravin tying an Adams.

Here goes:

Tie in one set of barbules in the tail position with two snug, but not too snug, wraps. Take the second set and place them atop the first set so that the tips are even. Pinch together both sets with the left thumb and forefinger.

Undo the wraps. Now you have the barbule tips even and ready for tying.