<p>In many countries, pre-service secondary teacher education programs maintain strong disciplinary boundaries in their design and delivery. Future teachers of secondary school mathematics take courses on mathematics content in the university’s mathematics department and courses on mathematics pedagogy in the education department, with little or no connection being made between these two forms of knowledge by the people who teach the separate courses. In this paper, we are interested in the experiences of mathematics teacher educators (MTEs) who do make these connections, by crossing the boundary between their respective disciplinary communities. We call these people “hybrid” MTEs. The aim of this paper is to explore how boundary crossing enables identity formation of hybrid MTEs. Analysis of semi-structured interviews with three hybrid MTEs working in Australia and the Czech Republic allowed us to compare the identity trajectories and boundary practices enabling them to connect, merge, or move between the communities of mathematics and mathematics education. Our research contributes to advancing knowledge in the field of mathematics education identity research. In addition, we argue that teacher education programs need to give more attention to enabling and encouraging collaboration between the mathematicians and mathematics educators who prepare future teachers.</p>