Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve 1 cup of pasta water before draining. Drizzle the cooked pasta with 1 tablespoon of olive oil to prevent sticking and set aside.

Heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Add the sliced Andouille sausage and cook for 5-7 minutes, turning occasionally, until browned and slightly crispy. Remove the sausage with a slotted spoon and set aside, leaving the rendered fat in the pan.

Add the peeled and deveined shrimp to the skillet, sprinkle with 1/2 tablespoon of Cajun seasoning, and cook for 2-3 minutes per side, or until pink and cooked through. Do not overcook. Remove the shrimp from the skillet and set aside with the sausage.

Reduce the heat to medium. Add the butter to the skillet. Once melted, add the diced onion, red bell pepper, and celery (the "holy trinity"). Sauté for 5-7 minutes until the vegetables have softened. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the sautéed vegetables and cook, stirring constantly, for 1 minute to create a roux. Gradually whisk in the chicken broth, scraping any browned bits from the bottom of the pan. Bring to a gentle simmer, then reduce heat to low. Stir in the heavy cream and the remaining 1/2 tablespoon of Cajun seasoning.

Add the cooked sausage, shrimp, and drained pasta to the skillet with the sauce. Toss everything together to coat. Stir in the 1/2 cup of grated Parmesan cheese. If the sauce is too thick, add a splash of the reserved pasta water until it reaches your desired consistency. Season with salt, black pepper, and an optional pinch of cayenne pepper to taste.

Garnish with fresh chopped parsley and additional grated Parmesan cheese, if desired. Serve immediately.