This study presents a hybrid optimization approach for the design of a two-stage helical gearbox with a split output stage, aiming to minimize the gearbox volume while maximizing transmission efficiency. A multi-objective genetic algorithm (NSGA-II) is employed to generate a set of Pareto-optimal solutions based on critical design variables, including gear ratios and face width coefficients. Subsequently, the Simple Additive Weighting (SAW) method is applied to identify the best compromise solution according to predefined decision criteria. The proposed approach accounts for geometric constraints, power transmission requirements, and mechanical losses to ensure both structural compactness and high performance. Simulation results demonstrate that the hybrid NSGA-II–SAW method effectively balances the conflicting objectives, providing a practical tool for optimal gearbox design in industrial applications.