Preheat the oven to 400°F.
In a large bowl, mix the cherries with 2 tablespoons lemon juice. In a smaller bowl, mix the blueberries with the remaining 1 tablespoon of lemon juice. Combine the sugar and cornstarch; gently stir 1 cup of the mixture into the cherries and ¼ cup into the blueberries.
Place 1 uncooked pie crust in a pie pan. Place the other pie crust on a cutting board and cut out some stars and stripes.
Spoon the cherry mixture over ¾ of the pastry in the pan, leaving 1 quadrant free. Spoon the blueberry mixture into the unfilled area. Scatter the butter evenly over fruit.
Top the filling with stars and stripes. Stripes over the cherries and stars over the blueberries. Set the pie in a foil-lined 13x17-inch baking pan.
Bake on the bottom rack until the fruit is bubbling in the center and the pastry is golden brown, about 1 hour. If the pie crust is browning too quickly, gently cover it with foil.
Let the pie cool on a rack for at least 3 hours. It's best served the same day. Serve slices with whipped cream or vanilla ice cream to make this a red, white and blue dessert!