Preheat oven 400 degrees F.
Place potatoes directly on rack in center of oven and bake for 30 minutes; pierce each potato in several spots and continue to bake until tender, about 30 minutes. Allow potatoes to cool slightly, trim off the top of potatoes to make a canoe-like shape. Carefully scoop out most the potatoes into a bowl, taking care to leave the skin intact.
Mash the potatoes lightly with a fork. Add the sour cream and buttermilk. Fold in the scallions and crumbled bacon. Season with salt and pepper. Refill the shells with the potato mixture, mounding slightly. Sprinkle with the cheese, return to the oven and bake for additional 20 minutes until heated thoroughly; serve immediately.