The role of conveners in initiating and/or leading the formation of cross-sector partnerships (CSPs) has received ample attention in the literature on partnership formation. However, what happens when two organizations jointly take up this role? This question is important in many complex partnerships, such as those on sustainable supply chains. We present a qualitative, longitudinal case study explaining how two organizations together convened the formation of a complex cross-sector partnership, and emphasize the changing roles of each organization individually and their shared relationship over time. We analyze how this process of ‘collective convening’ unfolded, how it impacted the collaboration, and how the conveners coped with the tensions brought about by the constellation of collective convening. These tensions manifested at the interorganizational level in the relationship between the two conveners, but were also fed by each convener’s organizational level dynamics, as these dynamics influenced the collective convening process. A paradox perspective is appropriate to analyze the tensions emerging from the collective convening process and the ensuing organizational response strategies. We contribute to the literature on CSPs by introducing and elaborating on the notion of ‘collective convening’, which reflects a reality in complex CSP formation but has remained largely unstudied in the existing literature on convening. Further, we show how conveners bring their organizational level dynamics to the interorganizational level and how these dynamics influence the CSP formation process in positive and negative ways.