Preheat the oven to 400 degrees . In a medium bowl, whisk the mascarpone with 1 teaspoon salt and 1/2 teaspoon pepper.
Arrange half of the red potatoes in a greased 9-by-13-inch baking dish, then top with half of the sweet potatoes. Spread half of the mascarpone evenly on top. Repeat with the remaining potatoes and mascarpone. Wrap tightly with foil and bake until the potatoes are tender, 30 to 35 minutes.
In a medium bowl, combine the breadcrumbs, parmesan and chopped scallions; stir in the melted butter. Sprinkle on top of the potatoes. Bake, uncovered, until golden, about 15 minutes. Sprinkle with the scallion greens and let stand for 10 minutes before serving.