HEAT oven to 350°F.
PREPARE cake batter and bake as directed on package for 2 (9-inch) round layers. Cool cakes in pans 10 min.; invert onto wire racks. Gently remove pans. Wash and dry cake pans; reserve 1 for later use. Cool cakes completely.
MEANWHILE, add boiling water to gelatin mix in small bowl; stir 2 min. until completely dissolved. Stir in cold water. Pour 1/3 cup gelatin into 9-inch round pan sprayed with cooking spray. Refrigerate 10 min. or until gelatin is set but not firm. Top with fruit then remaining gelatin. Refrigerate 30 min. or until gelatin is firm.
DIP knife in warm water, then run around edge of pan to loosen gelatin from side of pan. 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. (Do not remove gelatin from pan.) Spread 1/2 cup COOL WHIP over gelatin; cover with 1 cake layer, rounded side up.
PLACE remaining cake layer, rounded side up, on serving plate; spread with 1 cup of the remaining COOL WHIP. Invert gelatin-filled pan over frosted cake layer; remove pan. Spoon remaining COOL WHIP into resealable plastic bag. Cut hole in one bottom corner of bag; use to pipe COOL WHIP into spaces between layers.