Step 1: Cook the fettuccine according to package directions in a large pot of salted boiling water. Before draining, reserve 1 cup of the starchy pasta water. Drain the pasta and set aside.
Step 2: While the pasta cooks, heat olive oil and butter in a large skillet or Dutch oven over medium-high heat. Add the shrimp and scallops to the hot pan in a single layer. Sear for 1-2 minutes per side until pink and cooked through. Do not overcrowd the pan; cook in batches if necessary. Remove seafood from the pan and set aside.
Step 3: Reduce heat to medium. Add the minced garlic to the skillet and sauté for 1 minute until fragrant, being careful not to burn it. Pour in the white wine (or broth) and scrape up any browned bits from the bottom of the pan with a wooden spoon. Let it simmer for 2-3 minutes until slightly reduced.
Step 4: Stir in the heavy cream and red pepper flakes (if using). Bring the sauce to a gentle simmer, then reduce heat to low. Cook for 3-5 minutes, stirring occasionally, until the sauce thickens slightly.
Step 5: Remove the skillet from the heat. Gradually whisk in the grated Parmesan cheese until fully melted and smooth. Season with salt and black pepper to taste. If the sauce is too thick, add a splash of the reserved pasta water until desired consistency is reached.
Step 6: Add the cooked fettuccine and the seared shrimp and scallops back into the skillet with the sauce. Toss gently to coat everything evenly. Serve immediately, garnished with fresh chopped parsley and extra Parmesan cheese.