HEAT oven to 225ºF.
BRING rhubarb, water and 2 Tbsp. sugar to boil in saucepan; simmer on medium-low heat 5 min. or until rhubarb is softened. Cool, then refrigerate until ready to use.
MEANWHILE, beat egg whites and cream of tartar in medium bowl with mixer on high speed 5 min. or until soft peaks form. Add remaining sugar, 1 Tbsp. at a time, beating until stiff peaks form. Spoon into 12 mounds on parchment-covered baking sheet. Use back of spoon to indent center of each mound.
BAKE 40 to 45 min. or until crisp, dry and very lightly browned. Cool completely. Meanwhile, beat pudding mix and milk in medium bowl with whisk 2 min. Stir in COOL WHIP. Refrigerate 15 min. or until slightly thickened.
PLACE meringues on dessert plates just before serving. Fill with pudding mixture; top with rhubarb.