Prepare cake batter and bake as directed on package for 2 (9-inch) round layers, blending dry gelatin mix into batter before pouring into prepared pans. Cool cakes completely in pans.
Pierce cakes with large fork at 1/4-inch intervals. Blend next 3 ingredients in blender until well blended; slowly pour over cakes.
Refrigerate 1 hour or until liquid is absorbed. (Liquid will be absorbed slowly by the cakes.)
Mix 1/2 cup COOL WHIP and lemon juice in medium bowl with whisk until blended; stir in remaining COOL WHIP. Stack cake layers on plate, filling and frosting with COOL WHIP mixture.