Boil a large pot of salted water and cook the 12 oz fettuccine until al dente, about 10 to 11 minutes. Reserve 0.5 cup pasta water, then drain and set aside.

Pat the 8 oz scallops dry. Heat 1 tbsp olive oil and 2 tbsp butter in a large skillet over high heat. Sear scallops 2 minutes on the first side and 1 minute on the second, then transfer to a plate.

In the same skillet over medium-high heat, cook the 1 lb shrimp about 2 minutes per side until pink and curled, then transfer to the plate with the scallops.

Reduce heat to medium. Melt the remaining 2 tbsp butter, add the 4 cloves minced garlic, 0.25 tsp garlic powder, 0.5 tsp salt, and 0.25 tsp black pepper. Stir 1 minute, then pour in 2 cups heavy cream. Simmer 4 to 5 minutes. Off the heat, stir in 1.5 cups parmesan a handful at a time. Loosen with reserved pasta water if needed.

Return skillet to low heat. Toss the fettuccine in the sauce for 1 minute, return the seafood for 30 seconds to warm through, then transfer to a serving bowl and scatter 2 tbsp chopped parsley over the top.