This flakie gradient was made using Inglot 204 (the blue flakie) and Finger Paints Flecked (the green flakie) over a base of OPI Black Onyx. You may have seen a similar manicure on More Nail Polish -- I love her three color, mattified version!
This look is an easy one to achieve. You apply each flakie color like you would a glitter gradient, concentrating one flakie color at each end of the nail. After you have applied both colors, grab the one you applied first and add a couple of extra flakes to the middle of your nail. This helps to give the flakies a more "blended" look.