Soak beans separately in small bowls for 4 hours or overnight; drain. Cook in separate pots, covering each with 2 inches of fresh cold water. Bring to a boil, reduce heat, and simmer, covered, until barely tender, about 1 hour. Don't overcook. Drain and cool beans by plunging them in cool water to prevent the skins from splitting. Drain and set aside.

Just before serving, combine onion, vinegar, oil, salt, sugar, lemon juice, black pepper, and cayenne pepper in a large bowl; mix well. Add beans, red pepper, and scallions; toss to blend. Divide among four lettuce-lined plates.