Preheat oven to 425 degrees. Cover sweet potatoes and russet potatoes each with 1 inch water in 2 separate medium saucepans. Bring to a boil, salt generously, and boil until fork-tender, about 8 minutes. Drain and return potatoes to respective pans.
In a bowl, combine milk, cream, and butter. Divide milk mixture between saucepans. Mash potatoes until smooth; season with salt and pepper.
Spread mashed sweet potatoes evenly in the bottom of a shallow 2-quart baking dish. Top with mashed russet potatoes; spread evenly to edges. Bake until golden, about 25 minutes.
Meanwhile, in a bowl, combine flour and cayenne. Add leeks; toss to coat. In a medium saucepan, heat 2 inches oil over medium-high heat until a thermometer registers 350 degrees. Working in batches, add leeks and cook, turning, until crisp and light golden around edges, about 1 minute. With a slotted spoon, transfer to paper towels to drain. Season with salt.
Sprinkle fried leeks over potatoes; serve.