Co-teaching is fundamental to inclusive education. However, the way co-teaching is implemented, varies considerably, and establishing and allocating instructional responsibilities does not follow a standardized pattern. This study is based on four cases -- two located at traditional secondary schools and two at newly created community schools -- and includes semi-structured in-depth interviews with four regular teachers and four special education teachers. It aims at providing insight into how co-teaching partners in Germany perceive their own and their partner's instructional responsibilities. Data are analyzed with a structuring technique of qualitative content analysis. It becomes evident that the partners in each case do not always follow identical instructional intents. Nonetheless, different types of teacher knowledge seem to complement each other effectively in co-teaching relationships. Although, of course provided primarily for pupils with special needs, special education expertise also seems to support pupils without such needs. The results indicate a need to secure and integrate co-teaching relationships structurally into a school's development process. The article concludes with some options for development in practice.