Preheat oven to 400°F (200°C).
Scrub the potatoes. Pierce them several times with a fork.
Melt 2 tbsp of butter and brush it over the russet potatoes. Sprinkle with salt.
Place potatoes on a baking sheet and bake for 50-60 minutes, until tender.
Cool potatoes slightly. Cut russet potatoes in half and scoop out the insides, leaving a thin layer.
Peel and mash the sweet potato separately.
In a large bowl, mash the russet potato filling with 4 tbsp butter, sour cream, ½ cup cheddar cheese, milk, garlic powder, salt, and pepper.
Gently fold the sweet potato into the russet mixture, creating a marbled effect.
Refill the potato skins with the marbled mixture. Top with the remaining cheese.
Bake for an additional 15-20 minutes, until the cheese is melted and golden.
Top with optional toppings like green onions, butter, bacon, or sour cream. Serve hot and enjoy!