Preheat oven to 375° with rack in middle. In a large bowl, mix oils. Scrub and dry potatoes. Rub each thoroughly with oils and sprinkle generously all over with salt.
Bake directly on oven rack until skins are very crisp and a fork slides in easily, 1 1/2 to 2 hours.
While potatoes are still warm, score an X in top of each. Using a kitchen towel, squish potatoes from sides into center so that soft, steamy potato flesh bursts up through the X. Into each X, nestle 1 tbsp. butter, then top with 2 tbsp. sour cream. Sprinkle with chives.
Make ahead: Up to 2 hours at room temperature. Recrisp skins on a hot grill (or under broiler), turning often, 5 to 10 minutes.