Grease and flour 2 (8- or 9-inch) round pans. Prepare cake batter as directed on package; pour half into medium bowl. Add dry gelatin mix; stir until blended. Spoon half the white batter and half the pink batter, side by side, into each prepared pan; swirl gently with spoon.
Bake 30 min. Cool cakes 30 min. in pans. Remove to wire racks; cool completely.
Mix sour cream and sugar in medium bowl until blended. Gently stir in COOL WHIP. Stack cake layers on plate, filling with 1 cup of the COOL WHIP mixture and 1 cup berries. Frost top and side with remaining COOL WHIP mixture. Top with remaining berries.