Preheat oven to 350. Prepare a 9 x 13" pan with cooking spray; set aside. In a double boiler, add chocolate chips, butterscotch chips, and margarine. Heat until melted and smooth. Stir in graham cracker crumbs until mixture forms coarse crumbs. Reserve 1 cup crumb mixture for topping. Press remainder into prepared pan. Bake for 12 minutes. Meanwhile, to prepare filling, combine cream cheese, sugar, egg whites, flour, and lemon juice. Pour evenly over baked crust. Sprinkle with reserved crumb mixture. Bake for 25 minutes more.