Nowadays systems are becoming more and more connected. Consequently, the co-engineering of (cyber)security and safety life cycles becomes paramount. Currently, no standard provides a structured co-engineering process to facilitate the communication between safety and security engineers. In this paper, we propose a process for co- engineering safety and security by the explicit systematization and management of commonalities and variabilities, implicitly stated in the requirements of the different standards. Our process treats the safety and security life cycles as members of a security-informed safety-oriented process line and so it forces safety and security engineers to come together and brainstorm on what might be considered a commonality and what might be considered a variability. We illustrate the usage of our process by systematizing commonalities and variabilities at risk analysis phase in the context of ISO 26262 and SAE J3061. We then draw lessons learnt. Finally, we sketch some directions for future work.