Preheat the oven to 400 and scrub the potatoes clean.
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. Leave the shells intact.
Scoop most of the flesh into another bowl. Add the sour cream and butter and mash.
Stir in the green onions, 1/2 cup cheese, the nutmeg, 1 1/4 teaspoons 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. Place the stuffed potatoes on a baking sheet and bake until the cheese melts, 25 to 30 minutes.