Purpose: Co-teaching is a foundational mentoring model used in teacher residency programs in urban classrooms throughout the United States of America. Beyond the basic understanding of co-teaching in categorizing classroom models, the purpose of this qualitative case study is to investigate the dialectical tensions manifested in mentored co-teaching activities through the lens of cultural-historical activity theory (CHAT). Design/methodology/approach: Designed as a qualitative case study of 17 pairs of teaching-residents and mentor-teachers, the authors used thematic analysis to scrutinize archival interview data in an urban teacher residency program located in the largest megalopolis of the USA Northeast. The authors used CHAT-based concept coding to analyze the interview narratives from participants across different secondary school placements as they reflected on their co-teaching philosophy and the relationships they built. Findings: The authors found that for teaching-residents and mentor-teachers to co-develop as co-teachers, they jointly must learn to resolve the dialectical tensions of unbalanced classroom ownership vs added co-working responsibilities, breaking from routine so that a partnership can grow. Furthermore, the findings suggest that the prefix co- should be understood as (1) shifts in thinking that transcend the status quo and (2) the orchestration of human capital to change norms. Originality/value: This new understanding of the prefix co- allows teacher education programs to better mediate the dialectical tensions experienced by co-teachers in a mentored co-teaching activity, from individual teacher learning (e.g. a pair/dyad comprising one teaching-resident and one mentor-teacher) to collective co-learning across activity systems (e.g. partnership-based teacher education).