In this contribution, I reflect on how multilingual pedagogies can be added to STEM and S.T.E.M. practices, in what could be called a "double-crossing": on the one hand, crossing linguistic boundaries and, on the other, crossing specific subject knowledge. Based on a content analysis of teachers' interviews referring to their pedagogical experiences with a multilingual platform providing subject content in several languages, I reflect on the extent to which and how multilingual STEM and S.T.E.M. pedagogies are implemented and accounted for. More specifically, I analyse the arguments put forward by teachers for using or not using multilingual practices in STEM and S.T.E.M. and I delve into the apparent contradictions underlying the fact that, despite using a multilingual platform, its multilingual affordances are sometimes overlooked, underestimated or even used in monolingualised practices in multilingual settings.