Keep a close eye on these latkes while they are frying; the sugar in the sweet potatoes causes them to brown quickly.
In a large bowl, combine scallions and eggs. Add flour, salt, ginger, cardamom, and pepper, and stir until mixture is smooth. Add both kinds of potatoes, and toss until combined and evenly coated.
Fill a large, heavy-bottom skillet with about 1/2 inch oil. Place over medium heat until oil is almost smoking. (To test, drop a small bit of batter into the skillet; the oil should sizzle upon contact.)
Working in batches so as not to crowd skillet, carefully spoon about 2 tablespoons batter into oil for each pancake. Lightly tamp down to flatten. Cook until golden on each side, 2 to 3 minutes, turning once. Using a slotted spatula, transfer to drain on a paper-towel-lined wire rack, and repeat with remaining batter. Serve immediately with applesauce and sour cream, if desired.