Heat the oil in a medium Dutch oven over medium-high heat. Add the lemongrass if using, ginger and garlic and cook, stirring, until fragrant, about 2 minutes. Add the curry paste, fish sauce and sugar and cook, stirring, until the sugar dissolves, about 2 minutes.
Add the coconut milk and bring to simmer. Add the clams and mussels and reduce the heat, cover and simmer until the shells open, 5 to 7 minutes. Discard any unopened shells.
Stir in the basil, cilantro and lime juice. Divide the mussels and clams between two bowls, spoon the broth over them and serve with crusty bread.