Add the chicken broth and water into a pot on high heat. Add the lemongrass, galangal, kaffir lime leaves and roasted chili paste into the broth and bring to boil.
Add the chicken, shrimp, mushroom and red chili. When the chicken and shrimp are cooked, add fish sauce, coconut milk and lime juice. Stir to combine well. Turn off heat and serve immediately.
Notes
If you can't find lemongrass, galangal, and kaffir lime leaves, just skip them. There are no real substitutes but your soup will not have the aromas imparted by these ingredients. You can buy Thai roasted chili paste online.