Place the chickens, onions, carrots, celery, parsley, thyme, dill, garlic, salt and pepper in a 16- to 20-quart stockpot. Add chicken broth and bring to a boil. Simmer, uncovered, for 1 hour.
Remove the chickens and allow to cool slightly. Remove the breast meat from both chickens and set aside. Return the remaining chicken and carcasses to the pot and continue simmering, uncovered, for 3 more hours. Strain the entire contents of the pot through a colander and chill.
Remove the surface fat, then reheat the stock for Matzo Ball Soup, or pack in containers and freeze.