Cook pasta according to package directions, omitting salt and fat; drain.
While pasta cooks, heat 1 tablespoon oil in a large nonstick skillet over medium-high heat.
Add scallops and shrimp to pan; cook 3 minutes.
Remove scallop mixture from pan; keep warm.
Heat remaining 1 tablespoon oil in pan over medium-high heat.
Add onion, red pepper, and garlic; cook 2 minutes.
Add tomato paste and tomatoes; bring to a boil, and cook 2 minutes.
Add clam juice; cook 1 minute.
Add clams; cover, reduce heat to medium, and cook 4 minutes.
Add mussels; cover and cook 3 minutes or until clams and mussels open. Discard any unopened shells. Stir in scallop mixture and parsley; cook 1 minute or until thoroughly heated.