In a large bowl, combine flour and salt. Using a pastry blender, lightly cut the margarine into the flour until the mixture resembles coarse meal. Add water, a few tablespoons at a time, and mix lightly and quickly until pastry forms a ball. Divide pastry into two balls and form into two flat circles about 1 inch thick. Cover with plastic wrap and refrigerate at least one hour. On a floured surface, roll each ball into a 12 inch circle. (The pastry can be rolled between sheets of floured wax paper,or on a pastry sheet.) Place pastry in pie dish. Trim or flute the edges and prick the bottom of each shell lightly with a fork.