Preheat the oven to 425 degrees F. Prepare the pie crust and divide in half. Roll out the bottom crust and gently transfer it to a pie plate, trimming edges to fit.
Make cherry pie filling and cool completely. In a medium bowl, combine the blueberries, lemon juice, sugar, cornstarch, and butter.
Fold a piece of aluminum foil a few times to create a 2-inch wide strip. Bend the strip to create a right angle, then place it into the crust, creating a ¼ and ¾ division as shown in the pictures.
Spoon the blueberry filling into the ¼ section of the pie. Spoon the cherry pie filling into the other ¾ of the pie. Remove the foil dividing strip and discard.
Roll out the remaining pie crust dough. Cut out stars using a small cookie cutter or sharp knife, then cut remaining dough into strips using a pizza cutter or sharp knife. Decorate the top of the pie with the strips of dough and cut-out stars to represent the American flag. Flute the edges of the pie crust or crimp them with a fork.
In a small bowl, beat the egg with 1 teaspoon of water. Carefully brush the egg wash over the stars and stripes of the top crust, then sprinkle with coarse sugar, if desired.
Bake the pie for 20 minutes at 425 degrees F, then decrease the oven temperature to 350 degrees F and bake for another 30 to 35 minutes until golden-brown on top. Check the pie at about 20 minutes and cover the edges with a pie crust shield, if needed, to prevent them from getting too browned.
Cool for 3 to 4 hours before serving so the pie can set up.