Bring a large pot of salted water to a boil. Cook rigatoni until just shy of al dente, about 2 minutes less than the package says. Drain and set aside. The pasta will finish cooking in the oven.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Season shrimp with salt and pepper, then cook for 2 minutes per side until pink and just cooked through. Remove to a plate. Add remaining oil to the skillet, season scallops, and sear for 2 minutes without moving them until golden. Flip and cook 1 minute more. Remove from heat. The seafood will finish cooking in the bake.

In the same skillet, melt butter over medium heat. Add garlic and cook for 30 seconds until fragrant. Whisk in flour and cook for 1 minute, stirring constantly, until the raw flour smell disappears.

Gradually whisk in milk and cream, pouring in a steady stream while whisking to prevent lumps. Bring to a gentle simmer and cook for 5-7 minutes, stirring occasionally, until the sauce coats the back of a spoon. Remove from heat and stir in 1 cup Parmesan until melted. Taste and season with salt and pepper.

Preheat oven to 375°F. In a large bowl, toss the cooked pasta with the sauce, shrimp, and scallops. Transfer to a greased 9x13 inch baking dish. Top with mozzarella and remaining 1/2 cup Parmesan.

Bake uncovered for 20-25 minutes until the cheese is melted, bubbling around the edges, and starting to brown in spots. Let rest for 10 minutes before serving. Sprinkle with fresh parsley.