Research on professional learning and social networks among music educators points to the importance of closely-situated colleagues. However, the co-teaching dyad (i.e. a pair of music educators teaching elective instrumental music classes during the school day) has not been investigated in music settings. Research is needed on the co-teaching arrangement and how it interacts with advice-seeking. Therefore, the purpose of this case study was to investigate the co-teaching relationship among secondary (grades 6-12) instrumental music teachers and to understand the relationship of co-teaching with teachers' professional networks. Research questions were: (a) How do the participants interact around instructional issues? (b) How do the participants interact around administrative issues (e.g. micro-politics, resources)? (c) How does the co-teaching arrangement impact the participants' professional networks? Participants were two instrumental music educators who shared instructional responsibilities at a high school in the United States. Data sources included regular interviews and personal social networks created by the participants. Findings suggested that because the participants had a clear difference in experience, their co-teaching relationship functioned as a mentor/mentee dyad. However, the co-teaching arrangement combined with interpersonal trust to privilege novice teacher expertise in some situations. Instructional and administrative networks differed in meaningful ways between the participants.