Lightly coat a 3 1/2- or 4-quart slow cooker with cooking spray; set aside.
In a medium bowl combine flour, the 2/3 cup brown sugar, baking powder, the 1/2 teaspoon cinnamon, baking soda, and salt. In a small bowl combine eggs, melted butter, and milk. Add egg mixture to flour mixture; stir just until combined. Set aside.
In a large saucepan combine the 1/3 cup brown sugar and 1/4 teaspoon cinnamon. Stir in rhubarb and cherry pie filling. Cook and stir until mixture comes to boiling. Transfer hot fruit mixture to the prepared cooker. Immediately spoon batter on top of fruit mixture. In a small bowl combine granulated sugar and 1/4 teaspoon cinnamon; sprinkle on top of batter.
Cover and cook on high-heat setting (do not use low-heat setting) about 2 hours or until a toothpick inserted into center of cobbler comes out clean.
Remove liner from cooker, if possible, or turn off cooker. Let stand, uncovered, for 30 to 45 minutes to cool slightly before serving. To serve, spoon into dessert dishes. If desired, top with whipped cream and additional cinnamon.