In a large pot, bring salted water to a rolling boil over high heat. Add the fettuccine and cook according to package instructions until al dente, usually around 8-10 minutes. Reserve ½ cup of the pasta water for later and drain the rest. Set the drained pasta aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add the lobster and crab meat, sautéing for about 2-3 minutes until just warmed through. Remove the seafood from the skillet and set it aside.

In the same skillet, melt 2 more tablespoons of butter over medium-low heat. Add the minced garlic and sauté for about 30 seconds. Pour in the heavy cream, increasing the heat to bring to a gentle simmer. Stir in the Cajun seasoning and let it thicken for 3-4 minutes.

Reduce the heat to low and gradually stir in the grated Parmesan cheese until melted and the sauce is creamy. Return the sautéed lobster and crab to the skillet and add the reserved fettuccine. Toss gently to coat the pasta, adding reserved pasta water as needed.

Remove from heat, sprinkle with chopped fresh parsley and cracked black pepper, then serve immediately.