Process flour, salt, and sugar in a food processor until combined. Add the diced, COLD butter.
Process until the mixture resembles coarse meal, about 10 seconds. With machine running, add ice water in a slow, steady stream just until dough comes together. (Do not process more than 30 seconds.)
Turn dough out onto a piece of plastic wrap. Flatten and shape into a rectangle then wrap tightly in the plastic and refrigerate at least 1 hour (or overnight).
Preheat oven to 375 degrees. Toss peaches, raspberries, sugar, cornstarch, lemon juice, and salt in a large bowl; set aside.
Transfer one recipe of the dough crust to a floured work surface. Roll out dough into a 12-by-16-inch rectangle approximately 1/8 inch thick. Coat a baking sheet with cooking spray and then line with with parchment paper, leaving a 1-inch overhang on the sides. Transfer dough to the baking sheet. Trim edges of dough to hang over sides of baking sheet by 1/2 inch.
Roll out the remaining recipe of crust dough into another 12-by-16-inch rectangle. If desired, cut out small polka dots for decoration.
Pour fruit mixture onto the dough in the baking sheet.
Cover with remaining layer of dough.
Trim edges of dough to hang over sides by 1/2 inch. Fold edges of top layer over edges of bottom layer, and pinch to seal. Whisk together egg yolk and heavy cream in a small bowl.
Brush crust with egg wash. Sprinkle with sugar.
Bake until crust turns golden and filling is bubbling, about 1 hour and 10 minutes.
Let cool completely on a wire rack, about 3 hours. Pull up on parchment to remove.