The research presented in this paper aims to shed light on the complex role of mathematics teacher educators who led courses for prospective mathematics teachers. Theoretically, the research combines the documentational approach to didactics and the meta-didactical transposition frameworks, complementing the study with the analysis of the teacher educators' beliefs about the prospective teachers' needs. The methodology entails a comparative case study related to two courses, taking place in quite different contexts, led by the two authors independently, and a collaborative self-study of teacher education approach, based on eight reciprocal semi-structured interviews between the authors. The result is a holistic model of mathematics teacher educators' work that takes into account their practices, including the design of teaching materials based on a variety of resources, justifying discourses and personal beliefs. This can deepen our insight into the expertise of mathematics teacher educators and, prospectively, can be fruitful for their professional development.