Preheat oven to 350 degrees. In a small saucepan, boil with water. Add the celery and onion and cook 2-3 minutes, or until soft. In a medium bowl, toss cornbread crumbs, bread cubes, sage, parsley and poultry seasoning to mix well. Add chicken stock and egg whites and mix again. Pour the celery, onions, and cooking liquid into the bowl and mix well. Spray a 2-quart casserole with vegetable oil spray. Pour mixture into prepared casserole and dot with margarine. Bake, uncovered, 30 minutes, or until golden brown.