Mix first 5 ingredients in medium bowl. Rub in butter until mixture begins to clump together. Mix in nuts. (Can be made 1 day ahead. Chill.)
Preheat oven to 400. Combine apples, rhubarb, sugar, flour and vanilla extract in large bowl and toss to coat. Transfer apple mixture to 8x8x2-inch glass baking dish.
Sprinkle topping evenly over fruit. Bake until fruit is tender when pierced with knife and topping is crisp, covering with foil if topping is browning too quickly, about 45 minutes. Cool 20 minutes. Serve warm with vanilla ice cream.
9 Points Each(6 Servings); 6 Points Each(9 Servings);63 for Entire Recipe 8 Points Each(6 Servings); 5 Points Each(9 Servings);52 for Entire Recipe w/o Walnuts