Combine sugar and flour. Sprinkle about ⅓ of mixture in bottom of pastry-lined 9" pie pan. Turn mulberries and rhubarb into pie pan and add remaining sugar-flour mixture. Dot with butter. Adjust top crust, cut steam vents and flute edges.
Bake in hot oven (425°) 40 to 50 minutes, or until crust is browned a