This paper discusses an automated method for designing modular components that can be shared within multiple structural products, such as automotive bodies for sibling vehicles. The method is an extension of the concept of decomposition-based assembly synthesis. A beam-based topology optimization method, originally developed for First Order Analysis (FOA) of the automotive body structures, is utilized in order to obtain the “base” structures subject to decomposition. It is expected that the method will facilitate the early decisions on module geometry in automotive body structures, by enhancing the capability of the FOA system. Several case studies with two-dimensional structures are reported to demonstrate the effectiveness of the proposed method. The results indicate that two structures optimized for a similar, but slightly different boundary loading conditions are successfully decomposed to contain a component that can be shared by the structures. Several Pareto optimal decompositions are presented to illustrate the trade-offs among multiple decomposition criteria, with different weights for each objective function.