An apple pie is a balance between sweet and tart, crisp and tender. Adding cranberries upsets that balance- the tart berries overwhelm the sweet apples plus release juices that can make the bottom crust soggy. Arranging the fruits in two distinct layers allows the flavor of both to come through clearly. Use sweet crisp apples such as Golden Delicious, Jonagold, Fuji or Braeburn. The fillings may be made ahead, cooled and stored separately in the fridge for up to 2 days. The directions are very detailed as are all the recipes from Cook's Illustrated, my favorite magazine.