Shell and devein shrimp, reserving shells.
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.

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.
Add shrimp, fish sauce, lime juice and chilies.
Cook 2 to 3 minutes, or until shrimp are pink and curled.
Remove lemongrass.
Stir in green onions and cilantro and serve.