Coastal towns often protect their beaches from erosion by adding new sand in a process called beach nourishment. But when one town nourishes, the added sand can drift along the shore and help a neighboring town too. We built a model to explore what happens when two towns—one richer, one poorer—decide independently or together how often to nourish their beaches. We found that towns nourishing more often tend to lose sand faster and help their neighbors, even if those neighbors aren’t nourishing. A real-world example from New Jersey supports this: less wealthy towns often do more nourishment but get less benefit from it. Looking ahead, climate change and rising sand costs could make this worse. When towns plan together, they can do better overall—but sometimes the burden still falls on the town that values beaches more, even if it has fewer resources. Our study highlights the importance of cooperation between towns to protect shorelines fairly and sustainably in a changing climate.