Heat oven to 425°F. Prepare pastry with food processor or by hand. Place 4 ice cubes in a 1-cup measure and fill it with water. Fit food processor with metal blade and measure flour into bowl of processor. Add cubed butter and toss with hands to coat the cubes in flour. Pulse processor until butter pieces are pea-sized. Remove ice cubes from the water and pour out all but 1/2 cup water. Pour it through the feed tube of the food processor while pulsing. Stop when the mixture just starts coming together. It should still look dry and crumbly–overworking the dough is worse than underworking it. Turn the dough out onto a lightly floured surface. Gently push it together with your hands. Divide into 2 pieces and gently shape the pieces into flat round discs. Roll dough out and gently place bottom shell into pie dish. Reserve second disc for covering pie.
Stir together sugar, cornstarch and orange peel. Turn rhubarb into pastry-lined pie pan; sprinkle with half of the sugar mixture. Repeat with strawberries and remaining sugar mixture. Dot with butter. Cover with top crust and cut slits into it. Seal and flute edges. Bake for 40-50 minutes or until juice begins to bubble through slits in crust. Cover edges for last part of baking to prevent too much browning.
Makes about 8 servings.