Heat the olive oil in a skillet over medium-high heat; cook and stir the garlic in the oil until golden brown; add the tomatoes and mix well until juicy; stir in 1 teaspoon oregano. Lie the cod and shrimp into the mixture; season with salt; cover and simmer 3 minutes. Flip the cod and season again with salt and 1 tablespoon oregano; re-cover and cook another 3 minutes. Remove the cover and allow to cook until the juice evaporates slightly, 2 to 3 minutes.