Preheat oven to 350°F (175°C).

Prepare white cake batter according to package directions for 2 (9-inch) round layers.

Bake cakes as directed on package.

Cool cakes in pans for 10 minutes.

Invert cakes onto wire racks.

Gently remove pans.

Wash and dry cake pans; reserve 1 for later use.

Cool cakes completely.

In a small bowl, add boiling water to strawberry gelatin mix; stir for 2 minutes until completely dissolved.

Stir in cold water.

Pour 1/3 cup of gelatin into a 9-inch round pan sprayed with cooking spray.

Refrigerate for 10 minutes, or until gelatin is set but not firm.

Top with sliced strawberries and blueberries.

Pour remaining gelatin over the fruit.

Refrigerate for 30 minutes, or until gelatin is firm.

Dip a knife in warm water, then run it around the edge of the pan to loosen the gelatin from the side.

Dip the pan in warm water, just to the rim, for 15 seconds.

Lift from water and gently pull the gelatin from the edge of the pan with moistened fingers. (Do not remove gelatin from pan.)

Spread 1/2 cup of whipped topping over the gelatin; cover with 1 cake layer, rounded side up.

Place the remaining cake layer, rounded side up, on a serving plate; spread with 1 cup of the remaining whipped topping.

Invert the gelatin-filled pan over the frosted cake layer; remove the pan.

Spoon the remaining whipped topping into a resealable plastic bag.

Cut a hole in one bottom corner of the bag; use to pipe whipped topping into the spaces between layers.