Melt butter in a 12-inch nonstick skillet over medium-high heat.
Add onions and garlic; saut 1 minute or until tender.
Add shrimp and scallops; saut 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.