Bring stock and water to a boil in a large pan over high heat.
Add lemongrass, coriander roots, galangal, tomatoes, lime leaves, lime juice, tamarind water, and red chillies.
Return to a boil then simmer for 15-20 minutes.
Season with fish sauce and palm sugar to reach a balanced salty-sour-hot-sweet flavor.
Remove from heat and transfer 220ml of stock to a separate pan.
Boil the transferred stock and cook the chicken chunks for 2 minutes.
Add prawns and cook for another 2 minutes until opaque.
Divide the soup into two bowls and garnish with Thai basil, coriander, and lime wedges.