Safety and Security concerns are usually interlinked while building critical software-intensive systems of systems. Several efforts try to approach both domains of expertise to increase the overall reliability of the systems and reduce costs by an earlier detection of issues and trade-offs. Despite the growing number of co-engineering practices at different life-cycle stages, there is a lack on business justifications such as economic costs of their adoption. We report on using a cost model to evaluate the convenience (or not) of adopting co-engineering practices in two industrial case studies (space and medical devices). Simulation results with the collected data suggest an improvement in quality if any of the selected co-engineering practices are integrated while cost increases in one case but reduces in the other. We discuss the results but, as they cannot be generalized, the main contribution is on proposing the cost model for answering the title’s question.