Prepare the Crust: Combine flour, sugar, and salt in a food processor; pulse until combined, 2 to 3 times. Add butter and shortening; pulse until mixture resembles small pebbles, 3 to 4 times. Drizzle in 4 tablespoons of the ice water; pulse until mixture begins to clump, 3 to 4 times, adding remaining 1 tablespoon of water, 1 teaspoon at a time as needed. Transfer mixture to a lightly floured work surface; knead until mixture is incorporated, 3 to 4 times. Shape into a 7- x 5-inch rectangle; cover with plastic wrap, and chill 1 hour.

Prepare the Filling: Combine nectarines, cherries, sugar, flour, lemon zest, lemon juice, and salt in a bowl; toss well. Spread mixture into a lightly greased (with cooking spray) 11- x 8-inch baking dish. Preheat oven to 400°F.

Roll dough into a 12-inch square (dough will be thick). Cut dough into 16 (3-inch) squares. Arrange squares over top of fruit in patchwork fashion, slightly overlapping edges. Whisk together yolk and water in a small bowl. Brush top of dough with egg mixture. Bake in preheated oven 30 minutes.

Without removing pandowdy from oven, reduce oven temperature to 350°F. Bake until Filling is thick and bubbly and Crust is golden, 25 to 30 minutes. (Cover after 15 minutes, if needed, to prevent overbrowning.) Cool 30 minutes before serving. Serve warm or at room temperature.