1. To prepare crust, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, 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 overlapping plastic wrap. Chill 30 minutes.
2. Preheat oven to 400°.
3. Roll dough, still covered, into a 12-inch square. Fit dough, plastic wrap side up, into a 10-inch square 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 a piece of foil; arrange pie weights or dried beans on foil. Bake at 400° for 8 minutes. Remove pie weights and foil; bake an additional 3 minutes. Cool 5 minutes on wire rack.
4. Reduce oven temperature to 350°.
5. To prepare filling, combine brown sugar, cornstarch, lemon juice, and cinnamon in a large bowl. Add pears, tossing to coat. Spoon pear mixture in prepared crust. Bake at 350° for 45 minutes. Cool in pan on wire rack.
6. To prepare glaze, place preserves and orange juice in a small bowl, stirring to combine. Brush glaze over tart. Cool completely before serving.