Preheat the oven to 400 degrees F. Put the russet and sweet potatoes directly on the oven rack and bake 30 minutes. Pierce each potato a few times with a fork and continue to bake until tender, 15 to 20 more minutes for the sweet potato and 20 to 25 more minutes for the russet potatoes. Let cool slightly.
Peel the sweet potato. Transfer to a bowl and mash lightly with a fork until almost smooth. Trim a thin slice off the top of each russet potato, cutting lengthwise. Scoop most of the flesh into another bowl, leaving the shells intact. Add the sour cream and butter to the bowl and mash with a fork. Stir in the scallions, 1/2 cup cheese, the nutmeg, 1 1/4 teaspoon salt, and pepper to taste. Gently fold in the sweet potato.
Brush the potato skins with olive oil and season with salt and pepper. Generously fill with the marbled potato mixture and sprinkle with the remaining 1/4 cup cheese.
Reduce the oven temperature to 375 degrees F. Place the stuffed potatoes on a baking sheet and bake until the cheese melts, 25 to 30 minutes.