Preparation Heat oil in large deep skillet over medium-high heat. Add onion; sauté until tender, about 5 minutes. Add garlic; stir about 30 seconds. Add 3 cups clam juice and next 4 ingredients to skillet; simmer 5 minutes. DO AHEAD Can be made 4 hours ahead. Cool slightly. Refrigerate uncovered until cold, then cover and chill. Bring to simmer before continuing. Add shrimp, scallops, and 3 tablespoons cilantro to simmering broth, adding more clam juice to thin if necessary. Simmer until seafood is just opaque in center, about 3 minutes. Season with salt and pepper. Divide among bowls; sprinkle with remaining cilantro.
Preparation Heat oil in large deep skillet over medium-high heat. Add onion; sauté until tender, about 5 minutes. Add garlic; stir about 30 seconds. Add 3 cups clam juice and next 4 ingredients to skillet; simmer 5 minutes. DO AHEAD Can be made 4 hours ahead. Cool slightly. Refrigerate uncovered until cold, then cover and chill. Bring to simmer before continuing. Add shrimp, scallops, and 3 tablespoons cilantro to simmering broth, adding more clam juice to thin if necessary. Simmer until seafood is just opaque in center, about 3 minutes. Season with salt and pepper. Divide among bowls; sprinkle with remaining cilantro.