These single-serving desserts are small, but the flavors inside --blueberry and raspberry -- are big. The heart cutout (made with a cookie cutter) provides a peek at the filling.
Unfold piecrusts on work surface. Using top of a 3-inch aluminum pie tin as a template, add 1/4 inch all around and use a knife to cut out seven rounds. Make a vent in center of each round with a 1/2-inch heart-shape cutter. Set crusts aside.
Stir together berries, cornstarch, sugar, and lemon juice in a medium bowl. Divide mixture among seven 3-inch aluminum pie tins. Top each with a crust; use fingers to crimp sides so they stop just before edge of tin. Beat egg with 1 teaspoon water. Brush crusts lightly with egg wash; sprinkle with sugar. Freeze pies 20 minutes.
Preheat oven to 375 degrees. Bake until golden and juices are bubbling, about 25 minutes. Let cool completely on a wire rack.