Cook the fettuccine according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3 minutes until translucent.
Stir in the minced garlic, red bell pepper, and green bell pepper. Cook for another 5 minutes until the peppers are tender.
Add the lobster meat and crab meat to the skillet, followed by the Cajun seasoning, paprika, salt, and black pepper. Stir to combine and cook for 2-3 minutes until heated through.
Pour in the heavy cream and bring the mixture to a gentle simmer. Let it cook for about 5 minutes, stirring occasionally, until the sauce thickens slightly.
Add the cooked fettuccine to the skillet, tossing to coat the pasta in the sauce. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.
Stir in the grated Parmesan cheese until melted and well combined.
Serve immediately, garnished with fresh parsley.