This paper presents a multi-objective optimization approach for the design of two-stage helical gearboxes featuring two parallel gears in the first stage. The primary goals are to minimize the bottom cross-sectional area of the gearbox, representing its physical volume, and to maximize the mechanical efficiency of the transmission system. To address the inherent trade-off between compactness and performance, the MOEA/D-DE (Multi-Objective Evolutionary Algorithm based on Decomposition with Differential Evolution) algorithm is employed. The optimization framework incorporates geometric, kinematic, and strength constraints, ensuring feasible and manufacturable solutions. Simulation results demonstrate that the proposed approach effectively explores the Pareto front, offering designers a range of optimal solutions balancing volume and efficiency. The findings highlight the potential of MOEA/D-DE in solving complex gear transmission design problems with competing objectives.