PREHEAT oven to 425°F. Mix rhubarb, strawberries, sugar and tapioca in large bowl. Let stand 15 minutes.
PLACE 1 of the pie crusts in 9-inch pie plate. Fill with fruit mixture; dot with butter. Cover with remaining crust; seal and flute edge. Cut several slits in top crust to allow steam to escape.
BAKE 45 minutes or until juices form bubbles that burst slowly. Cool.