In a heavy-bottomed saucepan, bring 4 1/2 cups of salted water to a boil.
Add the polenta in a slow, steady stream while stirring with a whisk or wooden spoon. Continue stirring for 30 seconds. Reduce heat to low, cover, and cook for 5 minutes.
Remove from heat and keep covered for 5 more minutes. Stir in the cheeses. Cover until ready to serve.