For the shellfish: Heat 3 tablespoons olive oil in a skillet over medium heat. Add garlic; cook and stir until starting to sizzle, about 2 minutes. Stir in scallops and cook for 1 minute. Add shrimp and season with salt, pepper, and red pepper flakes. Cook over high heat until shrimp are bright pink on the outside and the meat is opaque, 3 to 5 minutes.

Pour 1/2 cup white wine into the skillet and let the alcohol evaporate. Scrape the bottom of the skillet to deglaze; cook until the liquid is reduced, 2 to 3 minutes. Remove from heat, add lemon juice, and sprinkle with 1/2 of the parsley. Discard garlic. Cover the skillet to keep shellfish warm.

To make the risotto: Pour fish stock into a saucepan over medium heat and bring to a simmer.

Melt butter in a skillet over low heat. Add 1 tablespoon olive oil and shallot. Cook until shallot is soft and translucent, but not brown, 3 to 5 minutes. Increase heat and add rice; cook until rice has absorbed the oil and butter and has a nutty aroma, stirring often, about 1 to 2 minutes. Make sure not to burn the shallot and reduce the heat if the skillet gets too hot.

Pour 1/2 cup white wine into the skillet and stir until alcohol has evaporated. Add 1 ladleful of fish stock, stir, and cook until rice has absorbed the stock. Add another ladleful and repeat the process until rice is tender, 15 to 18 minutes in total.

Add shellfish with their juices to the risotto pan 3 minutes before the end of cooking; stir and cook until shellfish are warmed through, adding stock if needed. Remove from heat. Stir in remaining parsley and 1 tablespoon extra-virgin olive oil. Let risotto stand briefly before serving.