Fill a large pot with lightly salted water and bring to a rolling boil. Cook fettuccine at a boil until tender yet firm to the bite, about 8 minutes.
Meanwhile, melt butter in a large nonstick skillet over medium-high heat. Stir in green onions and garlic; cook for 1 minute. Stir in shrimp and scallops; cook for 3 minutes, then reduce the heat to medium-low.
Pour in half-and-half and season with salt and pepper; stir constantly and bring to a simmer, making sure it does not boil. Gradually stir in 1/2 cup Parmesan; cook and stir for 1 more minute, then remove from the heat.
Drain pasta. Add to the skillet and toss with sauce until thoroughly coated. Sprinkle remaining Parmesan over top and serve immediately.