Soak the bean threads in a large bowl of hot water until soft, about 15 minutes; drain and cut into bite-size lengths. Divide the noodles into 4 bowls.
Bring the chicken broth, garlic, ginger, and lemongrass to a boil in a large saucepan; reduce heat to medium-low and cook the mixture at a simmer until fragrant, about 15 minutes. Add the chicken and shrimp to the soup; simmer until the chicken pieces are cooked through, 3 to 5 minutes. Stir the lime juice and fish sauce through the soup. Ladle the soup over the noodles; top with jalapeno pepper slices and cilantro to serve.