Once the dough has chilled overnight and has the texture and consistency of saltwater taffy, it's ready to be rolled. Liberally sprinkle a work surface with flour. Unwrap one of the dough disk and place it on the work surface, sprinkling its top with flour. If the dough is hard, let it rest for a few minutes. If the dough seems too sticky at first, add flour liberally. Use a rolling pin to roll with light pressure, from the center out, moving the pie dough around every quarter to roll it evenly. Roll the the dough to about 11 to 12 inches in diameter and about 1/4 inch thick (place your pie plate upside down over it to check its size). Move the dough into the pie plate by folding it into quarters, then moving it into the plate and unfolding it. When the dough is in the plate, press it firmly into the bottom and sides of the plate. Use kitchen shears to trim the excess dough.
Next, arrange the fruit in the pie. Take the blueberry filling and arrange the fruit so that they only take up a quarter of the pie — this will be the quarter of the pie that you will top with stars. Fill the remaining three-quarters of the pie with the cherry filling. Return to the refrigerator while you make the stars and stripes lid.
Remove the second dough disk from the refrigerator and roll it out onto the floured surface into a circle about 11 to 12 inches in diameter. Use a pastry wheel or a sharp knife to cut out a quarter of the circle, similar to how you would cut a slice of pizza. Use a star-shaped cookie cutter to stamp out stars and arrange them over the blueberry filling of the pie. Use the pastry wheel to cut the remaining three-quarters of the circle into even stripes (I used a ruler because I'm a nerd). Arrange them over the cherry filling of the pie, lining the stripes accordingly with an American flag. Use a fork to crimp the stripes into the border of the pie. Transfer to the freezer to chill for an hour, preferably overnight.
When the pie has frozen, it's ready for baking! Center a rack in the oven and preheat to 350 (F). Line a baking sheet with parchment paper and set aside.
In a small bowl, whisk together 1 large egg and 1 teaspoon water to create an egg wash for the pie. Use a pastry brush to brush the mixture over the stars and stripes portion of the pie, avoiding the filling and the pie edge. Sprinkle 2 tablespoons of demerara sugar evenly over the egg wash, and place the pie on the baking sheet. Bake in the preheated oven for at least 1 hour, or until the crust has turned golden and the fruit filling is juicy and bubbling. Transfer to a wire rack and allow to cool to room temperature before slicing. Enjoy!