In a mixing bowl, combine corn bread, wheat bread, crackers, eggs, buttermilk, margarine, sage, and sour cream. Bring broth to a boil in a 2-quart saucepan. Add onions and celery. Simmer over medium heat for 15 minutes, or until vegetables are tender. Add broth mixture to ingredients in bowl and mix thoroughly. Put into a large, shallow baking dish. Bake at 350-degrees F. for 40-50 minutes, or until golden-brown and set in the middle. When a straw inserted at the center comes out clean, the dressing is done.