Existing software-defined SDx architectures highly depend on a centralized control plane and hence can face substantial reliability challenges in software-defined coalition (SDC) settings, in which the centralized control plane can be weakly connected to the data plane, or even disconnected from the data plane due to high dynamicity. On the contrary, distributed control planes (e.g., OLSRv2) provide autonomy but lose flexibility and global policy guarantees. In this paper, we present Magnalium, a novel system to achieve high reliability in SDC networks by composing multiple control planes in real-time. Magnalium introduces a novel, unified composition framework that uses a distributed verification to systematically generate forwarding rules in accordance with desired policy requirements. Magnalium also introduces several supporting components to address challenges in wireless environment and resource management. We conduct data-driven simulations, showing that Magnalium benefits from both centralized and distributed control planes and even reduces downtime by 65% over the most reliable individual control plane.