The simultaneous emission of two α particles—double-α decay—represents a long-predicted but unobserved mode of nuclear radioactivity. Here, we formulate this process as a genuine three-body problem within the hyperspherical coordinate framework and evaluate decay probabilities by numerically solving the corresponding hyperradial Schrödinger equation combined with large-scale random sampling of the potential parameters. Inspired by this, we demonstrate that the decay width ratio between simultaneous and sequential α emission exhibits a strikingly linear dependence on ZQαα-1/2, extending the barrier penetration dynamics into the correlated few-body regime. The nuclei Xe108, Ra218, Pu224, U222, Rn216, and Th220 are suggested as the most promising candidates for the observation of double-α decay, with predicted half-lives potentially accessible within present detection limits. Our results provide a unified framework for multi-α decay and open a pathway to probing nuclear clustering and few-body correlations in heavy nuclei.