This study presents a systematic decomposition process to carry out assembly synthesis as a tool during the conceptual design phase of a product. Two configurations obtained by structural topology optimization are decomposed automatically into assemblies consisting of multiple members with simpler geometries. The optimal decomposition can be posed as a graph partitioning problem, which is actually a discrete optimization task. Considering the nonlinearity and the corresponding computational overhead of the problem, a steady-state genetic algorithm is employed as the optimization method. The final objective function attempts to find a solution that brings about two structures with maximum structural strength, maximum assemblability, and one or more components that can be shared by both products. The software implementation is carried out and a typical problem is solved using the procedure. It is observed that the algorithm manages to find an acceptable solution, allowing the commonality of one component in both end products and still maintaining a good structural strength and assemblability.