12 cups peeled and sliced fresh peaches
3 cups fresh blueberries
1 cup granulated sugar
2 tablespoons fresh lemon juice
1⁄4 cup cornstarch
1 (14.1-ounce) package refrigerated piecrusts
1 large egg, lightly beaten
1 tablespoon turbinado sugar
Preheat oven to 350°F. In a large bowl, combine peaches, blueberries, granulated sugar, and lemon juice. Let stand for at least 10 minutes.
Drain fruit mixture in a colander; discard liquid. Transfer to a large bowl; stir in cornstarch. Spoon mixture into 13x9-inch baking dish.
On a lightly floured surface, unroll one piecrust. Lightly brush with water; top with second piecrust. Roll to 1⁄4 inch thick. Cut dough into 11⁄4-inch-wide strips. Arrange over filling in a lattice design. Trim excess dough extending past edges of dish. Brush lattice with beaten egg, and sprinkle with turbinado sugar.
Bake until top is golden brown and filling is bubbly, 45 to 50 minutes. (Loosely cover with foil to prevent excess browning, if necessary.)