1. Boil potatoes separately, then simmer until fork tender (approximately 10 min).
2. Remove 1/4 cup sweet potato liquid and 1/3 cup white potato liquid.
3. Beat white potatoes, 1/2 cream cheese, salt and pepper and reserved liquid until smooth. Repeat with sweet potatoes.
4. In shallow 2 1/2 qt baking dish, alternately dollop white and sweet potatoes by cupfuls. Cut and twist with knife to create marbled look.
5. Bake covered 350 degrees for 40 minutes.