Preheat the oven to 425 degrees F. Pierce the potatoes with a fork or paring knife a few times on all sides. Place on a baking sheet and bake until fork-tender, 60 to 80 minutes. Let cool slightly (but keep the oven on).
Meanwhile, melt the butter in a small skillet over medium heat. Add the garlic and shallot and cook until softened but not browned, about 2 minutes. Set aside.
Once the potatoes are cool enough to handle, cut them in half lengthwise. Using a spoon or melon baller, scoop out the flesh, leaving a 1/4-inch rim to preserve the shape of the shells.
In a large bowl, mash the potato insides until mostly smooth. Add the butter mixture, sour cream, chives and 1 cup cheese and mix together. Season with salt and pepper to taste.
Spoon the mixture evenly into the 8 potato shells. Top with the remaining 1 cup cheese. Bake until the cheese is golden brown and bubbly, 25 to 30 minutes. Top with more chives. Serve warm.