letters for EB1b(OR) and NIW!

I want to file GC application for both EB1b(OR) and NIW, my question is how I handle the recommendation letters? Do I need 2 sets of letters from 2 different groups of recommenders or just ask each of recommender write me 2 letters(one for Eb1b and another one for NIW)? Did anybody have such experience? Your suggestion is really appreciated. Thank U in advance!