Once the sauce has heated up, add the Cream and Cheeses. Stir well. Once sauce has come to a simmer again, cover most of the way with a lid, and turn the heat down to Low. Make sure there is space for steam to escape. Let simmer for 2 - 3 hours, stirring every 20 - 30 minutes. Make sure your utensil is flat enough to scrape the bottom of the pan, as to prevent any burning.