Place the chicken breast halves in a large saucepan with enough water to cover. Bring to a boil. Cook 1 hour, or until the meat is tender and easily shredded. Shred chicken and set aside. Reserve 4 to 6 cups of the remaining broth.
Prepare an 8x8 inch pan of cornbread according to package directions. Crumble the corn bread into a large bowl. Mix in the white bread.
Preheat oven to 350 degrees F (175 degrees C).
In a medium saucepan over medium heat, melt the margarine and stir in the onions and celery. Slowly cook, stirring occasionally, until tender.
Stir the onions and celery into the bread mixture. Mix in the chicken, 4 cups reserved broth, cream of chicken soup, garlic powder, poultry seasoning, pepper and eggs. Blend with a potato masher until the mixture is the consistency of gelatin. Use more of the reserved broth as necessary to attain desired consistency. Transfer to a 9x13 inch baking dish.
Bake in the preheated oven 30 minutes, or until golden brown.