Preheat oven to 350. Prepare two 9" cake pans with cooking spray and flour; set aside. In a mixing bowl, combine cake mix, one and half ounces whipped topping mix, one cup milk, egg whites, and oil. Mix until moistened. Pour evenly into prepared pans. Bake for 30 minutes. Meanwhile, to prepare pumpkin frosting, combine another one and half ounces whipped topping mix, pudding mix, and remaining milk. Mix slowly until well blended. Then, beat again for eight minutes or until soft peaks form. Fold in pumpkin in carefully. Split cooled cake layers and frost each layer with pumpkin frosting. Stack layers on serving plate. Prepare remaining one and half ounces whipped topping mix according to package directions. Frost sides of cake. Sprinkle walnuts over top.