01 - Bring a large pot of generously salted water to a rolling boil. Add the fettuccine and cook according to package directions until al dente. Reserve 1/2 cup of the starchy pasta water before draining. Set the drained pasta aside.
02 - Pat the chicken pieces and shrimp dry with paper towels. Season both lightly and evenly with salt and freshly ground black pepper on all sides.
03 - Heat 1 tbsp olive oil in a large skillet over medium-high heat until shimmering. Add the seasoned chicken pieces in a single layer and sauté for 5 to 6 minutes, turning occasionally, until golden brown and cooked through to an internal temperature of 165°F. Transfer the chicken to a plate and tent loosely with foil.
04 - Add the remaining 1 tbsp olive oil to the same skillet. Place the shrimp in an even layer and cook for 2 to 3 minutes, flipping once, until they turn pink, curl slightly, and become opaque throughout. Remove the shrimp and set aside with the chicken.
05 - Reduce the heat to medium. Melt the butter in the skillet, then add the minced garlic. Sauté for about 1 minute, stirring constantly, until fragrant and lightly golden — do not let the garlic brown.
06 - Pour in the heavy cream and bring to a gentle simmer, stirring frequently. Stir in the grated Parmesan cheese, lemon zest, lemon juice, red pepper flakes (if using), salt, and black pepper. Continue to simmer for 2 to 3 minutes, stirring continuously, until the sauce coats the back of a spoon and has slightly thickened.
07 - Return the seared chicken and shrimp to the skillet. Add the cooked pasta and toss everything together thoroughly, ensuring each strand is evenly coated. Add splashes of the reserved pasta water as needed to reach your preferred sauce consistency.
08 - Fold in the chopped fresh parsley and taste the dish. Adjust the salt and pepper as needed to balance the flavors.
09 - Divide among warmed plates or shallow bowls. Finish each portion with a generous sprinkle of freshly grated Parmesan and a lemon wedge on the side. Serve immediately.