Crumble all of the cornbread and biscuits (or white bread slices) into a large bowl. Add salt, pepper, eggs, onion, celery, butter, cream of chicken soup, and chicken stock. Stir until well-combined.
Pour dressing into two 9x13 baking dishes and bake until lightly browned, about 30 - 45 minutes. To test doneness, shake casserole dish lightly. If the center of the dressing moves, then the dressing is not cooked through in the center. Continue to bake until the dressing is set throughout.
You can pour dressing into two smaller casserole dishes to reduce cooking time to about 35 minutes. I like to do this for one pan with onions and the other without for those who do not like onions.Each 9x13 pan serves 12. If you only need enough to serve 12, you can either half the recipe for one 9x13 pan or make the full recipe, pour into 2 9x13 pans and freeze one to bake later.