Having two teachers work collaboratively in the same class has been suggested as a possible solution to several instructional challenges, including the inclusion of students with special needs in mainstream classrooms and as part of school-wide prevention models to increase student achievement. In this, shared responsibility between teachers is regarded as a prerequisite to successful co-teaching. However, few studies have investigated whether shared responsibility between teachers actually leads to improved student achievement. This mixed methods study investigates shared responsibility in a sample of 148 classrooms where two general educators worked collaboratively in literacy instruction through first and second grade. First, we analysed whether the degree of shared responsibility between the two teachers for planning, enacting, and evaluating literacy instruction predicted student reading when controlling for pre-reading skills at baseline. Second, we carried out in-depth individual interviews with six collaborative teacher dyads purposefully selected from high- and low-performing classrooms to investigate what characterized their sharing of responsibility. The results show that shared responsibility significantly predicts students' reading achievement. Further, the interviews reveal a surface level collaboration between coteachers in low-performing classes, yet a more profound level of collaboration with influence on key teaching decisions in high-performing classes.