In large bowl, combine strawberries, nectarines and 1/4 cup sugar; set aside.
Meanwhile, heat oven to 425°F. Grease two 8-inch round cake pans. In large bowl, combine flour, 1/4 cup sugar, baking powder and salt; mix well. Using pastry blender or fork, cut in margarine until consistency of coarse meal. Add milk; stir just until dry ingredients are moistened. Spread dough in greased pans.
Bake at 425°F. for 20 to 25 minutes or until golden brown. Cool 5 minutes; remove from pans.
To serve, split warm or cool shortcakes; cut each round into six wedges. Spoon fruit mixture and whipped topping between layers and over top of each shortcake.