Preheat the oven according to the cake mix package directions. Prepare the cake batter as directed on the box and spread it evenly in a greased 9 x 13-inch baking pan
Bake until the center is set and a toothpick inserted into the middle comes out clean. Let the cake cool completely
Prepare the red and blue gelatin separately. Dissolve each gelatin powder in 1 cup of boiling water, then stir 1/2 cup cold water into each mixture. Allow the gelatin to cool slightly while keeping it liquid
Use the handle of a wooden spoon to poke evenly spaced holes across the cooled cake. Do not poke all the way through the bottom
Slowly spoon the red gelatin into some of the holes and the blue gelatin into the remaining holes. Try to distribute the colors evenly across the cake
Cover the cake and refrigerate for at least 2 hours, allowing the gelatin to set and the cake to become moist
Spread the thawed whipped topping evenly over the chilled cake
Arrange the blueberries and sliced strawberries over the top in a simple red, white, and blue pattern
Chill for another 30 minutes before slicing. Serve cold.