Bring 4 1/2 cups of salted water to a boil in a heavy-bottomed saucepan.
Whisk the polenta to the boiling water in a slow, fine stream. Stir constantly to avoid lumps.
Reduce the heat to low. Cover the polenta and cook it for another 5 minutes.
Remove from heat and keep covered for 5 more minutes. Stir in the cheeses.
Cover until ready to serve.