Stab 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. Set aside.
Once the fully cooked potatoes are cool enough to handle, cut them in half lengthwise. Using a spoon or melon baller, scoop out the flesh of the potatoes, 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 of Cheddar and mix together. Season with salt and pepper to taste.
Spoon the mixture evenly into the 8 shells. Top with the remaining 1 cup cheese. Bake until the cheese is golden brown and bubbly, 25 to 30 minutes. Serve warm.