Thaw frozen cherries and raspberries in a colander over a medium bowl, reserving juice. Add enough cherry juice to measure 1-1/4 cups.
Let pie crusts stand at room temperature for 15 minutes. Unroll pie crusts. Ease 1 of the pie crusts into a 9-inch pie plate without stretching it. Do not prick pastry. Brush pie crust bottom and sides with the 2 teaspoons melted butter; set aside.
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, lemon juice and 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 and seal. Crimp edge as desired. If you like, brush top pie crust with milk and sprinkle with more sugar. To prevent overbrowning, cover edge of pie with foil.
Bake in a 375 degree F oven 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 for at least 2 hours before serving. If you like, serve with vanilla ice cream.