CAKE: Preheat oven to 350 degrees . Coat two 15 x 10-inch jelly-roll pans with nonstick cooking spray.
Pour lemon juice into 4-cup measuring cup; add enough water to equal 2-2/3 cups. In large bowl, beat cake mixes, egg whites, lemon zest, juice-water mixture and lemon extract to blend, about 3 minutes. Divide evenly between two prepared pans; smooth tops.
Bake until cake springs back when lightly touched, about 20 minutes. Let cool on wire racks 10 minutes. Place rack over each cake. Carefully invert cakes and racks and remove pans from cakes. Let cool to room temperature.
FROSTING: Beat butter and shortening in large bowl for 1 to 2 minutes, until fluffy. Slowly add confectioners' sugar, milk and lemon juice. Continue beating on low until well blended and good spreading consistency, adding more milk a teaspoon at a time.
DECORATIONS: Place one cake layer on serving tray. Reserve 1 cup frosting for piping. Frost layer with 1 cup frosting; top with second cake layer; frost top and sides.
Place reserved frosting in pastry bag with a star tip. Mark off a field for stars with toothpicks 6 inches across and 5 inches down. Fill in with blueberries. Place seven red stripes of raspberries on cake as shown, leaving equal spacing for 6 white stripes between them. Carefully pipe white frosting stripes between the rows of raspberries.
Refrigerate uncovered to set frosting, then loosely cover with nonstick aluminum foil. Refrigerate until ready to serve. Let stand at room temperature for about 20 minutes before serving.