In two 8-inch nonstick skillets or omelet pans, melt the remaining 2 tablespoons of butter over moderate heat. Pour half of the egg mixture into each skillet. Cook, tilting the pans and drawing in the edges of the omelets with a spatula as they set to allow the uncooked eggs to seep underneath, about 5 minutes. Spoon the crab over half of each omelet. Fold the omelets over, then slide them onto plates. Cut each omelet in half, sprinkle with cilantro and serve.