This paper reports on a semester-long co-teaching investigation in which three mathematics teacher educators in the USA themselves enacted a co-plan/co-teach credentialing model in the context of a mathematics content course for future elementary teachers. The mathematics teacher educators sought to examine the co-planning practices for improving the team teaching co-teaching strategy. Data were collected from the mathematics teacher educators over fifteen weeks, including twice-weekly journal entries and classroom observation protocols. Utilizing a first-person research study design, the mathematics teacher educators found that a co-planning strategy could be used to maximize resources and improve lesson cohesion during team taught lessons. In particular, the mathematics teacher educators used co-planning meetings to divide lesson responsibilities by learning goal or by task. In this novel approach to mathematics teacher educator training, we describe the co-planning strategy along with its expansion, implementation, benefits, and limitations, and argue for its use in co-teachers' co-planning repertoire.