Peel the potatoes and rinse them under cold water. Cut into quarters, put in a large pot and cover with water. Add the salt, bring to a boil and cook until fork-tender, 20 to 25 minutes.  Drain the potatoes and return to the pot. Stir to evaporate any excess water, then mash the potatoes until no large chunks remain. Add the sour cream, butter and milk and mash until smooth. Add half of each cheese and mash until melted and incorporated. Add the remaining cheese and mash until melted. Thin with milk, if needed, and add salt to taste. Top with the chives.