Cream butter, shortening and sugar until well blended, light and fluffy. Combine flour, baking powder and salt. Mix milk and wter. Alternately add dry ingredients and liquid ingredients to butter mixture. Stir in vanilla and lemon extracts. Whip the egg whites with cream of tartar until stiff. Fold egg whites into batter. Spread batter into three 9-inch greased round cake pans. Bake at 350 degrees for 20 to 25 minutes or until lightly golden. Cool in pans on rack for 10 minutes. Turn out onto racks and thoroughly cool. Stack with Lemon Filling and frost with White Satin Frosting (see recipes below).