PREHEAT oven to 350°F. Prepare cake batter and bake as directed on package for two 9-inch round cake layers. Cool in pans 10 min. Invert cakes onto wire racks; gently remove pans. Cool cakes completely.
STIR boiling water into dry gelatin mix in small bowl at least 2 min. until completely dissolved. Stir in cold water. Pour 1/3 cup of the gelatin into clean 9-inch round cake pan (used for one of the cake layers) sprayed with cooking spray. Refrigerate 10 min. or until gelatin is set but not firm. Top with fruit and remaining gelatin. Refrigerate 30 min. or until gelatin is firm.
DIP knife in warm water and run knife around edge of gelatin to loosen. Dip pan in warm water, just to rim, for 15 sec. Lift from water and gently pull gelatin from edge of pan with moistened fingers. Spread 1/2 cup of the whipped topping over gelatin. (Gelatin is still in pan.) Cover gelatin with one of the cake layers, rounded side up. Set aside.
PLACE remaining cake layer, rounded-side up, on serving plate. Spread top with 1 cup of the remaining whipped topping. Invert gelatin and cake; gently place over frosted cake layer, gelatin-side up. Spoon remaining 1 cup whipped topping into resealable bag. Cut hole in one of the bottom corners of bag. Use to pipe topping into spaces between layers. Store in refrigerator.