Prepare Pasta: Bring a large pot of salted water to a rolling boil over high heat. Add spaghetti pasta and cook until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, then drain the pasta and set aside.

Sauté Vegetables: In a skillet, melt butter over medium heat. Add chopped red bell pepper, yellow bell pepper, white onion, and celery. Sprinkle with Cajun seasoning and sauté for 5-7 minutes. Add garlic and cook for an additional 1-2 minutes.

Make the Sauce: Stir in flour into sautéed vegetables and cook for 1 minute. Gradually pour in reserved pasta water and heavy cream, stirring until thickened. Mix in Parmesan cheese until melted. Season with salt and black pepper.

Cook Seafood: In a separate skillet, heat olive oil over medium heat. Add seasoned shrimp and sauté for 3-4 minutes until cooked.

Combine Everything: Gently fold in crabmeat into the sauce. Add reserved spaghetti and toss everything together until well coated. Let it simmer on low for 1-2 minutes.

Serve: Serve immediately while warm and creamy. Garnish with parsley or green onions, and accompany with lemon wedges if desired.