Using 2 pastry blenders, blend the flours, sugar, shortening, butter and salt.
Whisk the egg, vinegar and water in a 2-cup measure and pour over the dry ingredients incorporating all the liquid without overworking the dough.
Toss the additional flour over the ball of dough and chill if possible.
Divide the dough into 2 disks. Roll out 1 piece of dough to make a bottom crust. Place into a pie dish. Put dish in refrigerator to chill.
Preheat oven to 425 degrees F.