Bring a large pot of salted water to a boil over medium-high heat. Add the fettuccine and cook until al dente, 8-10 minutes, or according to the package directions. Reserve ½ cup of the pasta water, then drain the pasta and set aside.
Pat the shrimp dry with paper towels and season them with half of the Cajun seasoning. Set aside.
To a large skillet over medium heat, add the oil. Once hot and shimmering, add smoked sausage. Cook for 2-3 minutes per side, or until golden brown and slightly crispy. Transfer the sausage to a plate and tent loosely with foil to keep warm.
In the same skillet used for the sausage, add the seasoned shrimp in a single layer. Cook for 1-2 minutes per side, or until the shrimp are cooked through. Remove the shrimp from the skillet and set aside with the sausage.
Add the butter to the same skillet and melt over medium heat. Add the garlic and cook for about 1 minute.
Pour in the heavy cream. Add the remaining Cajun seasoning, salt, pepper, and crushed red pepper flakes. Stir to combine, then bring the mixture to a gentle boil, stirring occasionally.
Reduce the heat to low. Simmer the sauce for 6-8 minutes, or until it begins to thicken slightly.
Add the parmesan cheese and stir until completely melted and smooth.
Return the cooked fettuccine, sausage, and shrimp to the skillet. Toss everything together to coat in the sauce. If the sauce is too thick, stir in a splash of the reserved pasta water.
Garnish with chopped parsley and serve hot.