Lightly coat a 3-1/2- or 4-quart slow cooker with cooking spray; set aside. In a small saucepan bring apple pie filling to boiling. Stir in cherries. Transfer apple mixture to prepared cooker.
In a medium bowl stir together flour, granulated sugar, baking powder, and salt. Add milk and melted butter; stir just until combined. Stir in the 1/2 cup walnuts. Pour and evenly spread batter over apple mixture in cooker, spreading evenly.
In the same small saucepan combine apple juice, brown sugar, and the 1 tablespoon butter. Bring to boiling. Boil gently, uncovered, for 2 minutes. Carefully pour apple juice mixture over mixture in cooker.
Cover and cook on high-heat setting for 2 to 2-1/2 hours or until a wooden toothpick inserted near center of cake comes out clean. Remove crockery liner from cooker, if possible, or turn off cooker. Cool, uncovered, for 30 to 45 minutes.
To serve, spoon warm cake and its sauce into dessert dishes. If desired, top each serving with Sweetened Whipped Cream and additional walnuts.