Place the cheeses and milk in a medium heatproof bowl and set it over the pan of water. Let the cheeses melt together, stirring every so often to combine them with the milk, until you have a smooth liquid, 10 to 20 minutes. If the sauce is ready before the polenta, set it to the side until the polenta is almost done, at which point you can reheat the sauce on the stovetop, while whisking, to ready it for serving.☞ TESTER TIP: If your cheese sauce is too thick, whisk in additional milk, 1 tablespoon at a time until you get a smooth sauce.