Thaw peaches and rhubarb, if frozen; do not drain. In a large bowl, combine peaches, rhubarb, the 1 cup sugar, tapioca, Grand Marnier or orange juice, and butter. Cover; let stand 15 minutes.
Meanwhile. prepare Butter Pastry. On a lightly floured surface, flatten one of the dough balls. Roll from center to edge into a 12-inch circle. Wrap pastry circle around rolling pin; unroll into a 9-inch pie plate.
Preheat oven to 375 degrees F. Spoon fruit mixture into pastry-lined pie plate. Trim pastry 1/2 inch beyond edge of pie plate. Roll remaining dough ball to a 12-inch circle; cut dough into 1/2 to 3/4-inch-wide strips. Weave the strips over fill for lattice crust. Press ends of strips into crust rim. Fold bottom pastry over strips, trimming strips as necessary; seal and crimp edge.
Combine beaten egg and 1 tablespoon water; brush over pastry. Sprinkle lightly with sugar.
To prevent overbrowning, cover edge of pie with foil. Bake for 25 minutes. Remove foil. Bake 25 to 30 minutes more or until top is golden and filling is bubbly. Serve warm.
In a large bowl, combine 2 1/2 cups all-purpose flour and 3/4 teaspoon salt. Using pastry blender, cut in 1/2 cup butter-flavored or plain shortening and 7 tablespoons butter until pieces are pea-size. Sprinkle 1 tablespoon cold water over part of the mixture; gently toss with a fork. Push moistened dough to side of bowl. Repeat using 1 tablespoon of cold water at t time, until all the dough is moistened (6 to 7 tablespoons total). Divide dough into two balls.