In a large saucepan over medium high heat, combine the chicken breasts, chicken broth, salt and pepper. Bring to a boil and reduce heat to low. Cover and let simmer for 30 minutes, or until chicken is no longer pink in the center and the juices run clear.
Remove the chicken and let cool. Pour the remaining chicken broth mixture into a measuring cup. Let stand; spoon off fat. Add enough canned broth to equal 1 cup if needed. Add half and half to the broth mixture to equal 2 1/2 cups. Cut chicken into 1/2 inch pieces.
In the same pan, melt butter or margarine over medium heat. Add the onion and celery. Saute, stirring, for 3 minutes. Stir in flour until well blended. Gradually stir in broth mixture and the herbs. Simmer, stirring constantly, until the sauce thickens and boils. (you will think this is TOO MUCH liquid, but it's not). Add the chicken and vegetables to the broth and stir to combine. Line a deep pie dish with one of the pastries and pour mixture in, almost to the top.
Cover mixture with second pastry, making sure to "seal the edges" by pinching together the top and bottom then pressing against the lip of the dish. Cut a few vent holes in the pastry then brush pastry with beaten egg. Use any leftover scraps from the pastries to make a design on top.
Bake at 400 degrees for 30 minutes or until the crust is golden brown and the filling is bubbling. Let cool for 10 minutes and serve.
When done, the inside ingredients will be thick and creamy.