Bake at 400° for 60-70 minutes or until tender. Set sweet potatoes aside.
Cut a third off the top of each russet potato; scoop out pulp, leaving skins intact.
Place pulp in a bowl; mash with 1/3 cup sour cream, milk, cheese, 2 tablespoons chives and 3/4 teaspoon salt. Set aside.
Cut off the tip of each sweet potato; scoop out pulp, leaving skins intact. Mash pulp with remaining sour cream, chives and salt. Stuff mixture into half of each potato skin; spoon russet potato filling into other half.
Place on a baking sheet.
Bake at 350° for 15-20 minutes or until heated through.