1. In a large bowl, mix cherries with 2 tablespoons lemon juice. In a smaller bowl, mix blueberries with remaining 1 tablespoon lemon juice. Combine sugar and cornstarch; gently stir 1 cup of the mixture into cherries and 1/4 cup into blueberries.
2. Spoon cherry mixture over 3/4 of the pastry in pan, leaving 1 quadrant free. Spoon blueberry mixture into unfilled area. Scatter butter evenly over fruit.
3. Top filling with stars and stripes as directed for flag pastry. Set pie in a foil-lined 13- by 17-inch baking pan.
4. Bake on the bottom rack of a 400° regular or convection oven until fruit is bubbling in the center and pastry is well browned, about 1 hour. If pastry edges brown too quickly (check after 40 minutes), cover loosely with foil.
5. Let pie cool on a rack at least 3 hours; let stand at room temperature up to 8 hours. Cut into wedges and serve with ice cream, if desired.
If using fresh cherries, buy about 1 3/4 pound; if using frozen, you'll need about 1 1/2 packages, 16 ounces each.