This article argues that internal substate dynamics can systematically account for the organization of intergovernmental relations (IGR) in dual federal systems. Whereas majoritarian executive-legislative relations tend to weaken the institutionalization of intergovernmental arrangements (IGAs), power-sharing executive-legislative relations tend to facilitate it. Two of the mechanisms at work serve to illustrate this point. Given oneparty majority cabinets, complete government alternations strongly alter actors’ interest constellations over time, thereby increasing the costs of maintaining stable cross-boundary IGR. Moreover, the heavy impact of a potential electoral loss induces politicians to shift blame to other governments, thereby undermining cross-boundary cooperation. Majoritarian dynamics also weaken integration between IGAs. Furthermore, integration is weakened by compulsory power-sharing structures unbridged by party ties. In contrast to noncompulsory party cooperation, such internal constitutional divides easily undermine the setup of strong interorganizational linkages.