HEAT oven to 350ºF.
PREPARE cake batter as directed on package; stir in 1/2 cup bananas and nuts. Pour into 13x9-inch baking dish sprayed with cooking spray.
BEAT pudding mixes, milk and water in medium bowl with whisk 2 min.; stir in remaining bananas. Pour over batter in dish; place on baking sheet.
BAKE 55 min. to 1 hour or until toothpick inserted in center comes out clean. Cool 20 min. (Sauce will thicken slightly as it cools.) Sprinkle with sugar. Serve warm.