This only works for fiber that has longer guard hair than the undercoat or down that you want to separate out.

Using hand carders (either cotton or wool) card the fiber smooth in a brushing motion. Doft all the fiber onto one card so all the fiber is caught along the bottom (area furthest from the handle). At this point, the guard hairs being longer than the down or undercoat should be able to pulled off. If you do this carefully, you will have something like combed top for the guard hairs to be spun worsted. What is then left in the carder is the down undercoat.