Despite decades of research supporting the pedagogic value of learners' plurilingual resources to their linguistic and academic development, pre-service teachers frequently arrive at university inculcated in 'target language only' practices underpinned by monoglossic ideologies. The challenge for teacher education is to productively disrupt quotidian beliefs about language beliefs and prompt reconsideration of future classroom practices. Drawing on the work of the Douglas Fir Group (2016), this paper explores the identities, beliefs and values of two student-teachers as they emerged over the length of an innovative English-German pedagogic project on plurilingualism. The project involved German student-teachers developing a language portrait project for Grade 6 students; student-teachers using project data for undergraduate assignments; and English MA students interviewing young learners about their language portraits via videoconference. The videoconference provided young learners further opportunities to use their plurilingual resources and MA students with data for assignments on identity and investment. Working with DFG's framework (2016), we examine the interplay of the meso- and macro-dimensions of the larger project's design and the sometimes contradictory indexing of values and identities within and across activities. Analysis reveals that design choices sometimes unintentionally reinforced linguistic ideologies inconsistent with the project's objectives, though these conflicts also led student-teachers to unexpected insights. We close with personal reflections on the implications of the first iteration of this design-based research project for the advancement of plurilingual pedagogies in teacher education.