In large bowl, stir together sugar, tapioca, salt and nutmeg. Add rhubarb and strawberries. Toss gently to coat fruit. Let mixture stand for 15 minutes.
Line 9 or 10-inch pie plate with half the pastry. Trim pastry to edge of pie plate. Pour fruit mixture into pastry-lined pie plate. Dot with butter. Cut slits in top crust and place pastry atop filling. Seal and flute edge. Bake in 375 degree oven for 45-50 minutes. Cool pie on rack before serving.