Cook the Pasta:
In a large pot of salted boiling water, cook the fettuccine according to package instructions. Drain and set aside, reserving 1/2 cup of pasta water.

Cook the Chicken and Shrimp:
In a large skillet, heat the olive oil over medium heat. Add the diced chicken and cook for 5-7 minutes, until golden brown and fully cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the shrimp and cook for 2-3 minutes on each side, until pink and opaque. Remove and set aside with the chicken.

Prepare the Alfredo Sauce:
In the same skillet, add the butter and garlic, cooking until fragrant (about 1 minute).
Pour in the heavy cream and chicken broth, and bring to a simmer. Let it simmer for 3-4 minutes, until the sauce thickens slightly.
Stir in the grated Parmesan cheese, salt, black pepper, and red pepper flakes (if using). Stir until the sauce is smooth and creamy.

Combine the Pasta, Chicken, and Shrimp:
Add the cooked pasta, chicken, and shrimp to the skillet, tossing everything together until well coated with the Alfredo sauce. If the sauce is too thick, add a little reserved pasta water to achieve your desired consistency.

Serve:
Garnish with chopped fresh parsley and extra Parmesan cheese if desired. Serve hot.