Bring a large pot of generously salted water to a boil. Keep it ready while you start the sauce.
Season the dry shrimp and calamari lightly with salt. Heat 1 tablespoon olive oil in a large skillet over medium-high. Cook the shrimp for 60 to 90 seconds per side, just until opaque; transfer to a plate.
Add another tablespoon oil and the calamari. Cook, stirring, for 60 to 90 seconds, just until the rings turn opaque and firm. Transfer to the plate with the shrimp.
Reduce the heat to medium. Add the remaining oil, garlic, and red-pepper flakes. Stir for 30 seconds. Pour in the wine or stock and simmer until reduced by about half.
Boil the pasta until 1 minute shy of the package's al dente time. Reserve 1 cup pasta water, then drain.
Add the pasta to the skillet with 1/2 cup pasta water, the butter, lemon zest, and 1 tablespoon lemon juice. Toss over medium heat until the sauce turns glossy and clings to the strands, adding more pasta water as needed.
Return the shrimp and calamari to the skillet for 30 to 60 seconds, only long enough to warm through. Fold in the parsley, season with black pepper, and adjust the lemon and salt before serving.