Contemporary global governance architecture has been described as fragmented, incoherent, and inefficient. The mandates of prominent international organizations (IOs) often overlap and coordination is regarded more as the exception than the rule. In this context, international institutions designed with an explicit coordination mandate hold significant promise. These assembled institutions (AIs), like the Global Environment Facility (GEF) and UNAIDS, formally incorporate IOs in their designs and seek to coordinate and bring coherence to their work in a given issue area. Although praised for their bold and innovative designs, little scholarly work has assessed the success of these institutions in fulfilling coordination mandates. The paper introduces AIs as an organizational form and sheds light on two challenges they face that provide barriers to coordination. First, concerns regarding status and material losses may cause relevant IOs to resist coordination despite ostensibly opting in to participate in the assembled institution. Second, the governing bodies of assembled institutions are weak relative to the prominent IOs incorporated in their designs. The home governing bodies of these institutions often do little to reinforce the importance of coordination, hampering performance. These challenges are illustrated in the case of the GEF and UNAIDS.