Prepare two 9x13 casserole dishes with nonstick cooking spray.
Cook the macaroni according to package directions, approximately 8 minutes in boiling water. Drain well.
In a large pot, melt the butter over medium heat. Add the flour and stir to combine, cooking for about 1 minute as you stir. Slowly add the milk, stirring constantly. Allow the sauce to thicken until it coats the back of a spoon. Add salt and pepper.
Stir three cups of each cheese and buffalo sauce into the white sauce. Mix until the cheese has completely melted and the sauce is smooth.
In a large bowl, toss together the pasta, sauce, and chicken, stirring until evenly blended. Divide between the two casserole dishes.
Top the casseroles with the remaining cheeses.
To bake, cook in a 400 degree preheated oven for 30 minutes.