In a pan, melt the butter and add the chopped cheeses. Add the milk a littke at a time and melt over low heat. Adjust the consistency of fondue by adding more milk to make it less dense or otherwise parmesan if it is too much liquid. Boil the pasta in salted water, drain and mix with the cheese fondue, then add more grated Parmesan. Mix the pasta with cheese sauce and serve with a generous sprinkling of pepper.