Platform federations represent an emerging model in which independent host platforms selectively integrate functionalities from source platforms through specialized federation modules, creating a networked architecture that differs from traditional platform approaches. Through an empirical analysis of browser extensions that integrate AI source platforms across Chrome, Firefox, and Edge this study investigates the structural characteristics and governance dynamics of platform federations. Our analysis reveals that platform federations create double vertical complementarities and exhibit decentralized governance structures that balance integration with autonomy. These findings extend platform theory by conceptualizing federation-specific externalities and providing practitioners with strategic insights into operating in increasingly interconnected digital ecosystems. This study introduces a heterogeneous graph-based framework for analyzing platform federations, offering both theoretical and methodological contributions to understanding this emerging phenomenon.