Heat 3 tablespoons olive oil in a skillet over medium heat. Add garlic and cook until sizzling, about 2 minutes. Add scallops and cook for 1 minute, then add shrimp and season with salt, pepper, and red pepper flakes. Cook over high heat until shrimp are bright pink, 3 to 5 minutes.

Pour ½ cup white wine into the skillet and let the alcohol evaporate, deglazing the skillet. Cook until reduced, around 2 to 3 minutes. Remove from heat, add lemon juice, and sprinkle with half of the chopped parsley. Discard garlic and cover to keep the shellfish warm.

Heat fish stock in a saucepan over medium heat until simmering.

Melt butter in a skillet over low heat. Add 1 tablespoon olive oil and cook shallot until soft and translucent, about 3 to 5 minutes. Increase heat, add Arborio rice, and cook until the rice absorbs the oil and butter, stirring often, about 1 to 2 minutes.

Pour ½ cup white wine into the skillet and stir until evaporated. Gradually add ladlefuls of hot fish stock while stirring constantly, allowing each addition to be absorbed before adding more. Continue for 15 to 18 minutes until rice is tender.

Add shellfish and juices to risotto pan 3 minutes before the end of cooking. Stir in, add more stock if needed, and cook until warmed through.