Preparation Preheat oven to 325°F. Butter and flour two 9x5x2 3/4-inch metal loaf pans. Line pan bottoms with waxed paper. Sift flour, baking powder, and salt into medium bowl. Using electric mixer, beat 3 cups sugar, butter, and shortening in large bowl until well blended. Add eggs 1 at a time, beating to blend after each. Beat in dry ingredients in 3 additions alternately with milk in 2 additions. Beat in lemon juice and lemon peel. Divide batter between prepared pans. Bake cakes until tester inserted into center comes out clean, about 55 minutes. Cool cakes in pans 15 minutes. Turn cakes out onto racks; peel off paper. Cool completely. Combine strawberries, blackberries, and remaining 1/3 cup sugar in small bowl and toss gently to blend. Let stand until juices form, at least 30 minutes and up to 2 hours. Cut 1 cake crosswise into slices. Serve with berries and whipped cream.