Coconut soup with shrimp and chicken

1/2 lb medium shrimp in thin shells
Water as needed
2 cup chicken broth
14 oz can unsweetened coconut milk
2 lemongrass stalks bruised, and thickly sliced
4 oz fresh shiitake mushrooms stemmed, and caps sliced
1/2 lb boneless skinless chicken breasts cut crosswise into 1/4" strips
2 tbsp fish sauce
2 tbsp lime juice
2 fresh chiles seeded, and thinly sliced
2 green onions thinly sliced
2 tbsp chopped fresh cilantro
Shell and devein shrimp, reserving shells

1. Place shells with 2 cups water into a medium saucepan. Bring to a boil over medium-high heat. Reduce heat to medium and boil 5 to 10 minutes to extract flavor from shells. Strain broth and reserve.
2. In a large saucepan, combine chicken broth, shrimp broth, coconut milk and lemongrass. Bring to a boil, then reduce heat to medium. Add mushrooms and chicken and cook, uncovered, 4 minutes.
3. Add shrimp, fish sauce, lime juice and chiles. Cook 2 to 3 minutes, or until shrimp are pink and curled.
4. Remove lemongrass. Stir in green onions and cilantro and serve.