Arrange peaches in a buttered 13x9-inch baking dish. Combine strawberry, peach gelatin in a small bowl. Sprinkle about 3/4 of the mixture evenly over peaches and dot with butter. Prepare cake mix as directed on package. Pour 3/4 of the batter into the pan. Stir remaining gelatin mixture into remaining cake batter; blend well and pour into pan. Zig Zag spatula through batter to marble. Bake at 350 degrees F for 45 minutes, or until cake tester inserted in center comes out clean. Cool 5 minutes in pan, then invert onto serving platter and cool. Garnish with whipped topping.