Single-atom catalysts (SACs) have come to the forefront of heterogeneous catalysis, largely built upon the success of an isolated active site model that maximizes metal atom utility and offers excellent catalytic activity. However, the drive toward industrial relevance has spurred synthetic advances that now routinely yield high loadings, pushing SACs into a new high-density regime, where the conventional "isolated site" assumption no longer holds. This Perspective posits that intersite synergy is a tunable emergent property in these systems, shaped by the spatial distance between adjacent active centers. The impacts of site cooperation on heterogeneous catalysis manifest in various forms, including modified linear scaling relationships, alternating reaction pathways, and tunable spin-dependent activities. We conclude by outlining the challenges and opportunities in operando characterization and theoretical modeling of these cooperative catalytic ensembles, charting a course for the next frontier in the study of single-atom catalysis.