Thaw frozen cherries and raspberries in a colander over a medium bowl, reserving juices. Add enough cherry juice to equal 1-1/4 cups.
Preheat oven to 375 degrees F. Let pie crusts stand at room temperature for 15 minutes. Unroll pie crusts. Ease one of the pie crusts into a 9-inch pie plate without stretching it. Brush pie crust bottom and sides with the 2 teaspoons melted butter; set aside.
For filling, in a medium saucepan, combine sugar, cornstarch, tapioca, salt, and nutmeg. Add the 1-1/4 cups reserved juice. Cook and stir until thickened and bubbly. Remove from heat. Stir in the 2 tablespoons butter, the lemon juice, and the thawed cherries and raspberries.
Transfer the filling to the pastry-lined pie plate. Trim bottom pie crust to edge of pie plate. Cut slits in remaining pie crust; place on filling. Fold top crust under bottom crust and seal. Crimp edge as desired. If desired, brush top pie crust with milk and sprinkle with additional sugar. To prevent overbrowning, cover edge of pie with foil.
Bake for 30 minutes. Remove foil. Bake for 30 to 35 minutes more or until filling is bubbly and pie crust is golden. Cool on a wire rack. If desired, serve with ice cream. Makes 8 servings.