Trim the lemongrass, leaving 6 inches at the root end; discard the tops. Smash the stalks with the flat side of a knife and cut into 1-inch pieces.
Heat 2 tablespoons vegetable oil in each of 2 large pots over medium-high heat. Add 1 sliced onion to each and cook, stirring, until soft, about 5 minutes. Add half each of the garlic, lemongrass, ginger and curry paste to each pot and cook, stirring, until golden, about 2 minutes.
Add 1 can coconut milk, 2 tablespoons fish sauce and 1/4 cup water to each pot and bring to a simmer. Divide the mussels between the pots; cover and cook, stirring occasionally, until the mussels open, about 8 minutes. (Discard any that do not open.) Add the juice of 2 limes to each pot, then add the lime halves. Stir 1/2 cup cilantro into each. Serve with bread, if desired.