Preheat oven to 425F degrees. Cover 2 pounds sweet potatoes and 2 pounds russet potatoes each with 1 inch salted water in 2 separate medium saucepans. Add 4 garlic cloves to the russet potatoes. Bring to a boil and boil until fork-tender, about 10 minutes. Drain and return potatoes to respective pans.
In a quart measuring glass, combine ¾ cup milk, ¾ cup cream and 1 ½ sticks melted butter. Divide milk mixture between saucepans. Mash potatoes until smooth; season liberally with salt and pepper (about ½ teaspoon salt and ¼ teaspoon pepper in each pot).
Add 1 teaspoon Sriracha or hot sauce to the sweet potato mixture only.
Spread the 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 2 Tablespoons flour and ¼ teaspoon cayenne pepper. Add 2 leeks; toss to coat. In a medium saucepan, heat 2 inches of 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 and serve.