<p> How can states advance their claims in contested domains while mitigating the risk of war? Challenging the prevailing actor-centric focus, this study locates the answer in institutional statecraft: the employment of a procedural instrument—“Designing Duality”—to generate a strategic effect—“calibrated ambiguity”—and ultimately institutionalize a novel form of state power: “hybrid sovereignty.” Designing Duality is the systematic engineering of military-civilian, public-private, and formal-informal dualities to produce ambiguity as a strategic asset. A comparative analysis of maritime militias reveals two distinct design architectures. The Embedded Model (China, Vietnam), rooted in historically-constitutionalized path dependency, maximizes state control while entailing high attribution risk. Conversely, the Outsourced Model (Philippines, United States) delegates sovereign functions through compensatory-contractual bundling, creating political distance to achieve plausible deniability while accepting significant agency slack. This comparative analysis demonstrates a fundamental trade-off between control and deniability, forcing states into a strategic choice. Ultimately, this study establishes Designing Duality as a critical, yet overlooked, procedural instrument of modern statecraft. Thereby, states do not merely manage ambiguity but institutionalize it as hybrid sovereignty—a dynamic capacity to assert, retract, and obscure authority in contested domains. The underlying logic of this mechanism suggests its powerful applicability to other gray zones where international law is silent.</p>