Bring 6 cups water and salt to a boil in a large saucepan. Gradually stir in grits. Cook 4 to 5 minutes, stirring often, until thickened. Remove from heat. Add shredded cheeses, butter, and pepper, stirring until blended. Serve immediately.
Note: Grits may be chilled and reheated. Whisk 1/4 cup warm water into grits over medium-heat, adding more water as necessary.