PLACE cakes, top-sides up, in 2 clean 9-inch round pans. Pierce cakes with large fork at 1/2-inch intervals.
ADD 1 cup boiling water to each flavour jelly powder in separate bowl; stir 2 min. until completely dissolved. Carefully pour red jelly over 1 cake layer, and green jelly over remaining cake layer. Refrigerate 3 hours.
UNMOLD cakes. Fill and frost with Cool Whip. Keep refrigerated.