Beat egg whites and granulated sugar at medium speed with an electric mixer about 3 minutes or until foamy; add 1/2 cup butter, beating until blended.
Combine 2 1/2 cups flour, 1/2 teaspoon baking soda, and 1 teaspoon cream of tartar; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Set batter aside.
Beat egg yolks and brown sugar at medium speed with electric mixer; add remaining 1/2 cup butter, beating until blended. Stir in 1/2 cup molasses.
Combine remaining 2 1/2 cups flour, remaining 1/2 teaspoon baking soda, remaining 1 teaspoon cream of tartar, and spices; add to brown sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition.
Drop batters by spoonfuls, alternating dark and light next to each other, into 2 greased and floured 9- x 5-inch loafpans. Gently swirl light and dark batters together with a knife.
Bake loaves at 350° for 45 to 50 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes, and remove from pans.Cool loaves completely on wire racks.
Drizzle Orange Glaze over warm loaves; cool completely.