This study focuses on the effects of collaborative lesson planning by science pre-service teachers on their attitudes and knowledge. In our study, 120 pre-service teachers discussed a preparation for a science inquiry lesson in dyads. The teacher with the lesson preparation had the role of the coachee, while the other was the coach. We investigated the following research questions: (1) Does learning occur between the two peers? and (2) Is the competency in lesson planning affected by the attitude and knowledge of coach and coachee? Based on an actor-partner interdependence model (APIM), we could clarify the relations of pedagogical content knowledge (PCK) and attitudes (ATT) between and within the dyads of coach and coachee, as well as their development over time. Furthermore, the APIM allowed the inclusion of a mediator (lesson planning competency). Both PCK and ATT increased slightly but significantly during our project. ATT and PCK seemed to converge between coach and coachee at the end of the project. However, we could not find any cross-lagged effects, meaning there was no effect of coach on coachee or vice versa over time. Further, preceding PCK showed a significant effect on the competency of lesson planning, but planning competency did not influence succeeding PCK or attitude. Finally, these results are discussed with respect to science teacher education.