You don’t need a tart pan for this double-fruit dessert because the crust is simply folded up over the peaches and nectarines before baking. White peaches won't work as well in this tart because of their delicate texture.
Line baking sheet with parchment paper; coat parchment paper with cooking spray.
Preheat oven to 350°. Stir together nectarines, peaches, and next 4 ingredients in a large bowl. Let stand 30 minutes, stirring occasionally.
Unroll piecrust on prepared baking sheet. Roll into a 12-inch circle.
Drain fruit mixture, reserving liquid. Toss nectarines and peaches in flour.
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.
Stir together egg and 1 Tbsp. water. Brush piecrust with egg mixture, and sprinkle with 1 Tbsp. sugar.
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.
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.