Place flours, margarine, nutmeg, and salt in a food processor and process 10 seconds to a cornmeal texture. With processor running, add 3 Tbsp ice water. Add remaining water slowly, and stop the machine as soon as the dough begins to form a ball. Tiny pieces of margarine should be visible in dough.
Form the dough into 2 equal balls and flatten one into a thick disk. Flour your work surface and rolling pin, and roll dough from center out to sides, turning into a perfect circle about 10 inches in diameter. (Rolling the dough between two pieces of waxed paper works pretty well too. -- Ari)
Place the rolling pin in the center of the dough. Fold half over the pin and transfer to a 9-inch pie plate. Press the dough gently into the pie plate, fold the rough edges under neatly or trim them. Wrap second ball of dough in waxed paper and refrigerate (or wrap tightly and freeze if you're making a single-crust pie -- Ari). Cover pie plate with a slightly damp kitchen towel and place in refrigerator.
Preheat oven and prepare filling as specified for whatever filling recipe you're using. After pie is filled, if you're using the second crust, roll it to a 9-inch circle. Place it over the filling, trim, and crimp edge with thumb and forefinger or flute with a fork. Brush top with melted margarine if desired.