Pat steak cubes and shrimp completely dry with paper towels. Season both with 0.5 teaspoon salt and the black pepper. Bring a large pot of salted water to a boil for the pasta.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering. Add steak cubes in a single layer and sear undisturbed for 2-3 minutes until a deep brown crust forms. Flip and cook 2 minutes more for medium-rare. Transfer to a plate and tent with foil.

Add remaining 1 tablespoon olive oil to the same skillet. Add shrimp in a single layer and cook 1-2 minutes per side until pink and just opaque. Transfer to the plate with the steak.

Reduce heat to medium. Add 4 tablespoons butter to the skillet. Once melted, add garlic and red pepper flakes; cook 30 seconds until fragrant but not browned. Pour in wine and simmer 3-4 minutes until reduced by half. Stir in lemon juice and remaining 2 tablespoons butter until glossy.

While the sauce simmers, cook pasta according to package directions until al dente. Reserve 0.5 cup pasta water, then drain.

Return steak and shrimp to the skillet with any accumulated juices. Add cooked pasta and half the parsley; toss gently to coat, adding reserved pasta water 1 tablespoon at a time if needed to loosen the sauce. Serve immediately topped with remaining parsley and Parmesan if desired.