For the pasta: Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 8 to 10 minutes. Drain and transfer to a large serving bowl. Add the butter and parsley and toss until coated. Season with salt and pepper, to taste.
For the sauce: In a large skillet or saucepan, heat the oil over medium-high heat. Add the shallots and season with salt and pepper. Cook, stirring frequently, until soft, about 3 to 4 minutes. Add the garlic and cook for 30 seconds until aromatic. Add the wine and simmer for 2 minutes until the liquid has reduced by half. Add the broth, red pepper flakes, clams and mussels. Season with salt and pepper. Bring the mixture to a simmer. Cover the pan with a tight-fitting lid and cook until all the shellfish have opened, about 5 to 8 minutes. Discard any unopened shellfish.
Using tongs, remove the shellfish from the pan and reserve. Season the cooking liquid with 2 teaspoons tablespoon salt and 1 teaspoon pepper. Pour the shellfish cooking liquid over the pasta and toss well. Season with salt and pepper, to taste. Arrange thereserved shellfish on top of the pasta and serve.