Combine ½ cup melted butter and brown sugar in a large greased slow cooker; spread evenly across the bottom. Layer pineapple and peaches on top. Sprinkle with cider mix. In large bowl, combine cake mix, ¾ cup combined reserved juice and remaining butter. Spoon batter over fruit in slow cooker. Layer 6 paper towels under the lid of the slow cooker before closing it to absorb moisture. Cover; cook on low 2-3 hours or until a toothpick inserted in the center comes out clean. Cool slightly before serving. Scoop out individual servings.