Bring a large pot of salted water to a boil. Cook pasta until al dente. Reserve ½ cup pasta water, then drain.
Pat scallops and shrimp completely dry and season lightly with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat. Add scallops in a single layer and sear for 2–3 minutes per side until golden. Remove and set aside.
In the same skillet, add shrimp and cook for 1–2 minutes per side until pink and opaque. Remove and set aside.
Reduce heat to medium. Add butter and garlic to the skillet and cook for 30 seconds until fragrant.
Pour in chicken broth, scraping up any browned bits. Stir in heavy cream and bring to a gentle simmer.
Reduce heat to low and stir in parmesan cheese and red pepper flakes if using.
Add cooked pasta, scallops, and shrimp back to the skillet. Toss gently, adding reserved pasta water as needed to loosen the sauce.
Garnish with fresh herbs if desired and serve immediately.
