Place the cooked white and sweet potatoes in separate bowls. Add half each of the cream cheese and sour cream to the potatoes in each bowl; season with salt and pepper. Mash with potato masher or fork until creamy.
Stir half of the Parmesan cheese into the bowl of white potatoes. Stir half of the cheddar cheese into the bowl of sweet potatoes.
Alternately layer half each of the white potato mixture and sweet potato mixture in a 2-quart clear glass casserole dish. Repeat layers.
Bake 15 minutes. Sprinkle with remaining cheese and continue baking 5 minutes or until cheeses are melted.
Notes
Source: Adapted from KraftYou're Gonna Bake It After Allbakeitafterall.com