Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Melt butter in a large, non-stick skillet over medium-high heat. Stir in onions and garlic, and cook for 1 minute. Add shrimp and scallops, stirring to combine, and cook 3 minutes more. Reduce heat to medium-low.
Pour half-and-half, salt, and pepper into the pan and bring to a simmer, stirring constantly. Do not boil. Gradually sprinkle 1/2 cup Parmesan cheese over seafood mixture and continue stirring another minute. Remove from heat.
Toss cooked pasta into the pan, coating thoroughly. Sprinkle with remaining Parmesan cheese, and serve.