Formal de jure Standard Development Organizations (SDOs) coordinate the development of compatibility standards that ensure technological progress and welfare through interoperability. Informal SDOs, which are self-organized and not mandated by law, co-exist with formal SDOs to develop compatibility standards in a substituting or complementing way. As a result, the process of standardization evolves within and across multiple SDOs. An examination of the process of managing standardization across collaborating formal and informal SDOs can fundamentally change our theoretical understanding of governance of standardization. Drawing upon the tensions-based view, we argue that fundamental characteristics of the co-existence of complementary standardization are dynamic and contradictory. To balance such tensions, generative responses within and across formal and informal SDOs are required. This two-staged case study research examines the emergence of five tensions throughout the collaboration of two SDOs, HL7 and ISO, in health informatics. Our results explain the contradictory nature of five tensions with respect to (1) technological functions, (2) legal purview and geographic scope, (3) trigger, (4) time, and (5) usage/IPR and how they evolve over time. We also empirically derive three governance responses to such tensions, namely dynamic and multi-level roles, dynamic processes, and evolving resources; these responses allow members of the SDOs to respond to these tensions in a way that succeeds in managing the process of standardization. This research provides a novel theoretical understanding of governance of standardization for innovation. We conclude with recommendations for policy makers and standards developers.