Combine rhubarb, cherries, sugar, and tapioca; let stand 15 minutes.
Combine flour and salt in bowl; cut in shortening with pastry blender until mixture resembles cornmeal.
Sprinkle water over mixture; stir ingredients quickly with fork until blended; gather into a ball; divide in half.
Roll out one half to a 12-inch circle; fit into 9-inch pie plate. Trim even with edge of pan; moisten edge with cold water.
Spoon filling into pastry; dot with butter.
Roll out second half of pastry as the first; fold in half; cut designs or slashes in top to vent steam. Place over filling with fold at center; unfold. Trim upper crust 1/2 inch beyond edge of plate. Fold edge of top crust under edge of bottom crust. Flute with fingers.
Brush top of pie with milk and sprinkle with sugar. Bake in 425F oven for 45 to 50 minutes until crust is golden.