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.