1. Line baking sheet with parchment paper; coat parchment paper with cooking spray.
2. Preheat oven to 350°. Stir together nectarines, peaches, and next 4 ingredients in a large bowl. Let stand 30 minutes, stirring occasionally.
3. Unroll piecrust on prepared baking sheet. Roll into a 12-inch circle.
4. Drain fruit mixture, reserving liquid. Toss nectarines and peaches in flour.
5. Mound nectarines and peaches in center of piecrust, leaving a 3-inch border. Fold piecrust border up and over fruit, pleating as you go, leaving an opening about 5 inches wide in center.
6. Stir together egg and 1 Tbsp. water. Brush piecrust with egg mixture, and sprinkle with 1 Tbsp. sugar.
7. Bake at 350° for 45 minutes or until filling is bubbly and crust is golden. Carefully transfer tart on parchment paper to a wire rack; cool 20 minutes.
8. Meanwhile, bring reserved nectarine-peach liquid to a boil in a small saucepan over medium heat. Boil 1 to 2 minutes or until slightly thickened. Let cool slightly. Brush or drizzle 1 to 2 Tbsp. hot nectarine-peach liquid over exposed fruit in center of tart. Serve immediately with remaining nectarine-peach syrup, and, if desired, Sweet Cream Topping.