Boil a large pot of salted water. Add rotini and cook until al dente (about 8–9 minutes). Drain, drizzle with a little olive oil to prevent sticking, and set aside.
Toss shrimp and diced chicken separately with 1 tablespoon Cajun seasoning each until well coated.
Heat olive oil in a large skillet over medium heat. Add chicken and cook for 5–6 minutes until golden and fully cooked. Transfer to a plate.
In the same pan, melt 1 tablespoon butter. Add shrimp and cook 2–3 minutes per side until pink and opaque. Remove and set aside with the chicken.
Lower the heat. Add the remaining 1 tablespoon butter and minced garlic. Sauté for 30 seconds until fragrant. Pour in heavy cream and chicken broth, scraping the pan to lift any browned bits.
Stir in Parmesan and let the sauce simmer for 3–4 minutes until slightly thickened. Taste and adjust with salt, pepper, or more Cajun seasoning if desired.
Add pasta, chicken, and shrimp back to the skillet. Toss to coat evenly in the creamy sauce and cook for 1–2 minutes to blend flavors.
Turn off the heat. Sprinkle with chopped parsley and extra Parmesan if you like. Serve hot with garlic bread or a fresh green salad.