Heat the olive oil in a saucepan over medium heat. When the oil is hot, add the garlic and onion. Cover and cook over medium heat, stirring occasionally, until the onions and garlic are softened (about 1 minute).
Add the diced tomatoes (and their liquid),
tomato sauce, tomato paste, basil, oregano, red pepper flakes, and black
pepper. Bring to a boil, then reduce the heat to medium-low. Simmer uncovered
until the sauce is reduced (about 10 minutes).
While the sauce mixture is simmering, boil
the pasta in the large pot to your desired firmness. Drain well and set aside.
Add the shrimp, scallops, and olives to the
simmering sauce. Cook uncovered over medium-high heat until the shrimp are pink
and the scallops opaque (about 4 minutes).
Serve the pasta topped with the
shrimp-scallop mixture.