Heat oven to 375°F.
Place potatoes on rimmed baking sheet. Prick with fork, brush with 1 tablespoon oil, and season with ¼ teaspoon salt. Bake until tender and cooked through, 45 to 50 minutes.
Meanwhile, arrange bacon on small rimmed baking sheet; bake until crisp, 15 to 20 minutes. Transfer to paper towel–lined plate and crumble when cool.
Transfer potatoes to cutting board and let cool 10 min. Increase oven temp to 450°F. Slice potatoes in half lengthwise and scoop out flesh, leaving ¼ inch all around shells (reserve potato flesh for other use or use in Ranch Croquettes).
In small bowl, stir together 2 teaspoons ranch seasoning and remaining 2 tablespoons oil; brush onto inside and outside of each potato shell. Return potato shells to baking sheet and bake, flipping halfway, until crisped and browned, 15 to 20 minutes.
Fill shells with cheeses, scallion whites and light greens, and bacon and bake until cheese is melted, 5 minutes. Top with dark scallion greens.
Meanwhile, in bowl, combine sour cream and remaining tablespoon ranch seasoning. Serve with potato skins.
