HEAT oven to 350°F.
MELT 1/4 cup (4 Tbsp.) butter; mix with wafer crumbs. Press onto bottom and up side of 9-inch tart pan with removable bottom. Bake 10 min.; cool.
MEANWHILE, mix sugar and cornstarch in medium saucepan. Gradually add water, stirring with whisk until blended. Bring to boil on medium heat; cook 8 min., stirring constantly. Remove from heat. Add dry gelatin mixes, lemon zest and juice; stir until gelatin is completely dissolved. Add remaining butter; stir until melted. Refrigerate 30 min. or until slightly thickened, stirring occasionally. Pour into crust.
REFRIGERATE 3 hours or until firm. Remove tart from side of pan before serving topped with COOL WHIP.