Weigh or lightly spoon 5 ounces white rice flour (about 1 1/4 cups) and tapioca flour into dry measuring cups; level with a knife. Combine flours, 1 1/2 teaspoons granulated sugar, baking soda, and xanthan gum in a medium bowl; stir with a whisk. Cut in butter and shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Add milk; toss with a fork until moist. Gently press mixture into a 4-inch circle on 2 sheets of heavy-duty plastic wrap that overlap; cover with additional plastic wrap. Chill 30 minutes.
Preheat oven to 400°.
Roll dough, still covered, into a 13-inch circle. Fit dough, plastic wrap side up, into a 10-inch removable-bottom tart pan coated with cooking spray. Remove remaining plastic wrap. Press dough against bottom and sides of pan. Line bottom of dough with parchment paper; fill with pie weights or dried beans. Bake at 400° for 8 to 10 minutes; remove parchment paper, and cool on a wire rack.
Reduce oven temperature to 350°.
Combine 3/4 cup granulated sugar and cornstarch in a small bowl. Place cherries in a medium saucepan over medium-low heat; cook 5 minutes. Stir in sugar mixture. Reduce heat to low; cook 10 to 15 minutes or until mixture thickens, stirring frequently. Remove from heat; cool completely.
Weigh or lightly spoon 7 ounces white rice flour (about 1/3 cup) into a dry measuring cup; level with a knife. Place flour, brown sugar, and remaining ingredients in a food processor; process until coarsely ground.
Spoon cooled cherry mixture into crust. Sprinkle brown sugar mixture over filling. Bake at 350° for 40 minutes or until crust and topping are browned and filling is bubbly.