Safety and security engineering have been traditionally separated disciplines (e.g., different required knowledge and skills, terminology, standards and life-cycles) and operated in quasi-silos of knowledge and practices. However, the co-engineering of these two critical qualities of a system is being largely investigated as it promises the removal of redundant work and the detection of trade-offs in early stages of the product development life-cycle. In this work, we enrich an existing safety-security co-analysis method in the design stage providing capabilities for interference analysis. Reports on interference analyses are crucial to trigger co-engineering meetings leading to the trade-offs analyses and system refinements. We detail our automatic approach for this interference analysis, performed through fault trees generated from safety and security local analyses. We evaluate and discuss our approach from the perspective of two industrial case studies on the space and medical domains.The research leading to this paper has received funding from the AQUAS project (H2020-ECSEL grant agreement 737475). The ECSEL Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme