1. Preheat oven to 425°F. Mist a 2-quart baking dish with cooking spray. Line a rimmed baking sheet with foil. In a large bowl, combine peaches, plums, lemon juice, honey, tapioca and salt, mixing well. Let stand for 10 minutes, stirring occasionally.
2. Make topping: Combine flour, oats, sugar, cinnamon and salt in a large bowl. Add butter and use fingertips or a fork to mix until crumbly. Refrigerate until ready to use.
3. Transfer fruit mixture to baking dish. Sprinkle topping over fruit. Place dish on baking sheet. Bake until fruit is softened and topping is golden brown, 40 to 45 minutes. Cool slightly before serving.