MIAMI, FLORIDA.- The Whitney Museum of American Art is thinking of opening a branch in the arts district projected for downtown Miami. Maxwell Anderson, director of the Whitney, is thinking of using a warehouse space at the corner of Northeast Second Avenue and 13th Street -- 15,000 square feet, for this satellite location. Anderson was in Miami two weeks ago together with architect Richard Gluckman and they looked at this property. Gluckman has designed the Georgia O’Keeffe Museum in Santa Fe, N.M., and the Andy Warhol Museum in Pittsburgh.