Put the clams and about 1/2 cup water in a medium skillet. Cover and steam over high heat until the clams open, about 2 min. Discard any that don't open. Remove the clams and reserve. Strain the liquid from the pan and reserve. In the same skillet, wiped dry, heat 2 Tbs. of the oil over high heat. Add the scallops and sear on both sides until lightly browned, about 2 minutes; remove and reserve them. In the same skillet, heat the crushed garlic in the remaining 2 Tbs. oil until lightly browned. Add the tomatoes and red pepper flakes and simmer over moderate heat for 15 minutes.
Meanwhile, bring a large pot of water to a boil and add 1 Tbs. salt. Add the haricots verts and cook until just tender; remove with a slotted spoon and add them to the tomato sauce. (Keep the water boiling.) Add the scallops, the clams, and the strained clam liquid to the tomato sauce and cook until the sauce is slightly reduced, about 3 minutes. Stir in the parsley.
While the sauce reduces, stir the pasta into the boiling water and cover until the water comes back to a boil. Uncover and cook until the pasta just starts to become pliable but is still somewhat firm, about 2 minutes. With tongs, lift the pasta, let it drain for a moment over the boiling water, and add it to the sauce, tossing well, to finish cooking. When the pasta is al dente, transfer the pasta and sauce to a warm serving platter or individual bowls and serve immediately.