Melt butter over medium-high heat, then add shrimp and Old Bay seasoning
Cook for 3-4 minutes on both sides until cooked through. Add in the crab and stir.
To make cheese sauce, melt butter and whisk in flour in stages to prevent clumping.
Add the milk and heavy whipping cream and stir. Add half of the shredded cheese and the 4 ounces of cream cheese stir until the cheese is melted.
Add the onion powder, salt, and pepper to taste. Taste repeatedly and adjust seasonings as needed.
Place cooked pasta in a large pot. Drizzle the cheese sauce over the pasta and stir until fully combined.
Add in the cooked seafood and the butter sauce the seafood was cooked in.
Layer in pasta and cheese to a 9x13 baking dish, then bake for 25-45 minutes at 375 degrees F, until cheese is melted and bubbly.