Line a fine mesh strainer with a tea towel and add grated potatoes (sweet and russet) and onion. Squeeze out as much liquid as possible; discard. Transfer the mixture to a large bowl and add the eggs, flour, 3/4 teaspoon salt and 1/4 pepper. Use you hand to mix just until the mixture holds together, adding more flour if necessary.
Add enough oil to coat the bottom of a large non-stick skillet. Heat until the oil is hot, but not smoking. Working in batches, measure out 11/2-tablespoon portions of the latke mixture, flatten slightly and add to the skillet, spacing the latkes 1 inch apart. Cook until the latkes are crisp and brown around the edges, 4 to 6 minutes; flip and cook, until crisp and deep golden brown, 4 to 6 minutes more. Transfer to a paper towel-lined plate, sprinkle with additional salt and repeat with the remaining latke mixture, adding few tablespoons of oil between batches.
Meanwhile, whisk together the sour cream, parsley, chives, 1/4 teaspoon salt and 1/4 teaspoon pepper. Serve the latkes with the herbed sour cream.