This conceptual note introduces a dual-key legitimacy framework for security governance, designed to address escalation risk under conditions of technological acceleration, compressed decision timelines, asymmetric power shifts, contemporary armed conflict environments and hybrid warfare dynamics. The framework distinguishes between two irreducible sources of authority in collective security decision-making: population-level legitimacy grounded in civilian risk and humanitarian impact, and sovereign legitimacy grounded in state equality and legal responsibility. Rather than collapsing these sources into a single decision channel, the model treats them as independent legitimacy gates, each capable of constraining escalation. The framework is analytically distinct from veto reform, majoritarianism, responsibility-to-protect doctrines, and centralized global governance, and is intended to support both theoretical analysis and applied institutional design. It offers a generalizable conceptual architecture for escalation control that remains valid irrespective of specific institutional adoption. This publication is supplemented by a conceptual framework that formalizes the dual-key legitimacy model underlying the applied institutional design proposed here. The related work provides an abstract, generalizable account of conjunctive legitimacy constraints and escalation control, intended for theoretical analysis and cross-institutional application.