In large bowl, gently toss raspberries and cherries. Add 1 1/2 cups sugar, the tapioca and lemon juice. Mix well; let stand 1 hour.
Heat oven to 400°F. Place 1 pie crust in ungreased 9-inch glass pie plate. Spoon fruit mixture into pastry-lined pie plate; dot with butter. Cut remaining crust into 1-inch strips. To make top crust, place 5 to 7 strips pie crust across filling. Place cross-strips over tops of first strips. Crimp edges. Brush crust with milk; sprinkle with 1 teaspoon sugar. Cover edge of crust with 2- to 3-inch wide strips of foil to prevent excessive browning.
Bake 15 minutes. Reduce heat to 350°F; bake 40 to 45 minutes, removing foil after 10 minutes, until filling is bubbly and crust is golden brown. Cool completely on cooling rack, about 2 hours.