"This paper extends recent research work dealing with the analysis of “collective joint activities”. In these activities, actors belonging to different professional worlds have to fulfill different tasks in a coordinated way in order to reach an overall common goal. Such activities require the actors to operate not just sequentially, as distinct agents, but jointly, as a transverse team. A transverse team is defined as a group of actors who share a common view of the process as a whole, who understand their colleagues’ work and acknowledge their complementary contribution to the global task, and who are willing to improve their joint activity. This research develops a method - the collective co-analysis of practices - that encourages the development of a transverse team. The method seeks to allow actors to define their own collective activity, to better understand each-others’ activities, and to jointly elaborate new work practices. The method includes three steps. First, the existing work situation is analyzed in order to identify the joint activity, the obstacles to this joint activity, and the existence of a transverse collective. Second, spontaneous attempts to overcome the obstacles are identified in order to make work more collaborative. Third, cross-visits are organized. On day 1, agent A visits agent B; agent B describes his/her work while performing it; agent A asks questions and comments. On day 2, the situation is reversed: agent B visits agent A, etc. The method was applied in an organization in which a client-provider relationship between actors of a joint activity was enforced. This relationship was detrimental to the cooperative processes that were necessary to achieve a high quality of service. We then analyzed the first effects of the method, notably, the important role of collective reflections regarding the joint activity. These reflections led to a process of evaluation and transformation of the activity, and encouraged the emergence of a transverse team. The specificities and difficulties of collective joint activities lead us to consider necessary conditions for the emergence of a transverse team. Among these conditions, the existence of spaces for debate and confrontation between team partners appears to play a crucial role."