Combine shrimp shells with stock, chilies, salt, lime, zest, lime leaves and lemon grass stalks in heavy nonaluminum pot. Bring to a boil, reduce heat, cover and simmer 20 to 20 minutes. Strain.
Return liquid to pot and place over medium-high heat and bring to a boil. Add shrimp and scallops and cook 1 minute. Stir in fish sauce and juice of limes. Add chopped cilantro, slivered red chili and shiitake slices, and green onions. Stir and pour into a tureen or ladle into individual bowls.