taste and 1/2 of the garlic and puree until smooth. Reserve the remaining chipotle peppers and adobo sauce for another use. Transfer the pureed tomato mixture to a large bowl. Finely slice the cilantro and stir into tomato mixture. Add the red onion, salt and freshly ground black pepper. Cover bowl and refrigerate until chilled.
In a large skillet heat olive oil over moderately high heat. Add the shrimp and saute for 1 minute. Add the remaining garlic, stir to combine and cook for another minute. Add the clams and a splash of soy sauce. Add the remaining tomatoes and reserved juices, stirring to combine. Cover and cook until clams open, about 3 to 5 minutes.
Uncover the skillet, remove from heat and discard any unopened clams. Ladle about 1 cup of the chilled tomato mixture into the skillet and stir to combine. Stir the seafood mixture into the tomato mixture. Ladle soup into serving bowls and garnish with sliced scallions.