Melt butter in a 12-inch nonstick skillet over medium-high heat. Add onions and garlic; sauté 1 minute or until tender. Add shrimp and scallops; sauté 3 minutes or until done. Reduce heat to medium-low.
Add half-and-half, salt, pepper, and crabmeat; cook 3 minutes or until thoroughly heated, stirring constantly (do not boil). Gradually sprinkle 1/2 cup cheese over seafood mixture, stirring constantly; cook 1 minute, stirring constantly. Remove from heat. Combine pasta and seafood mixture in a large bowl. Top each serving with 1 1/2 teaspoons cheese and 1 1/2 teaspoons parsley.