Bring a large pot of water to a boil. Add the fettuccine, cooking pasta according to package directions (usually around 9 to 12 minutes). When done cooking, pour into a colander to drain the water, and set aside for now. Keep the pot you used set aside as well.
Meanwhile, heat a large non-stick skillet over medium heat. Add ½ tablespoon of butter. Season the salmon with ⅛ teaspoon salt and ⅛ teaspoon pepper. Place the salmon skin side down in the pan and cook for 4 minutes, then flip and cook for another 3-4 minutes, or until salmon is cooked through.* Set aside on a plate.
Add the other ½ tablespoon of butter to the same skillet. Season the shrimp with the Cajun seasoning, then add the shrimp to the pan. Cook for about 3-4 minutes, flipping halfway, until shrimp are cooked through (they should be opaque and pink). Remove and set aside.
In the empty pot that you previously used to cook the pasta, add the Neufchâtel cheese, milk, ⅛ teaspoon salt, and ⅛ teaspoon pepper to the pot. Heat over medium-low heat, stirring frequently, until creamy and well combined, about 3 minutes (do not boil).
Add the spinach to the pot with the sauce and cook for another 2 minutes, until the spinach is lightly wilted. Add the parmesan, continuously stirring, until it is mixed into the sauce, then immediately remove from heat.
Toss the cream sauce with the pasta. Portion out the salmon and shrimp and serve on top of the pasta. Add any additional parmesan as desired.