We analyse how two co-teaching mathematics teacher educators (MTEs) describe and justify the enactment of their theory of change in a course for practicing teachers. Applying concepts from Communities of Practice, we identify a shared view of the key boundary objects highlighted in the design of the course in our two MTEs, alongside divergent but complementary means of brokering learning at the boundary during enactment. Prominent in our analysis is a working relationship in which one MTE brokering through coordination appears to allow the other to work towards radical transformation, by seeking confrontation that allows her to define the problem space. We consider the implications of this dynamic for their emphasis on teaching as a pair.