One hour ahead: Heat oven to 350 degrees. Grease two 15.5 x 10.5 x 3 cm jelly-roll pans; line with wax paper; grease paper. Prepare cake batter according to package directions, working with one package at a time. Spread batter in prepared pans; bake 15 to 20 minutes until wooden toothpick inserted in center comes out clean. Invert cake layers onto wire racks; peel off paper; cool. In medium size bowl, toss banana slices with lemon juice. Pour off excess juice; set bananas aside. Slice about 24 well shaped strawberries lengthwise in half; reserve for top of cake. Coarsely chop remaining strawberries; set aside. In large bowl with electric mixer at high speed beat cream until soft peaks form; gradually beat in sugar until stiff. Spoon half whipped cream into second bowl; fold in chopped strawberries. Set both creams aside. Place one cake layer on large serving platter; tuck strips of wax paper under cake to keep platter clean. Using spatula, spread strawberry cream over cake; top with second cream cake layer. Spread plain cream over top and sides of cake; carefully remove wax paper strips. In top left hand corner of cake, using tip of sharp knife, mark 7.5 x 13 cm rectangle. Place blueberries in even rows in the rectangle to form stars of the flag. To from stripes, alternate rows of strawberries and bananas, overlapping fruit in each row as necessary. Chill until serving time, up to 2 hours. Cut into 2 x 4 cm pieces to serve.