Classically studied from independent methodologies and compartmentalized research programs, first person data (documenting the actor's personal experience from his own standpoint) and third-person data (data produced from the point of view of an outside observer, without reference to what the actor can feel and independently of his own point of view) have been braided together this past decade with a view to access a more complete and complex outlook on actions. How exactly has the field of French Sports Sciences contributed to the propagation of this original methodology consisting in confronting heterogeneous materials? Our epistemological analysis investigates the social and epistemic conditions of its genesis (progressive conquest of diversified subjects, reference to exemplar studies, dissemination from a core group of authors, etc.) until the establishment of an activity close to normal science. It also formalizes the diversity of the methods of joint analysis between these data (correlation, heuristic discordance, etc.) before evaluating the knowledge effects specifically generated (reinforcement of robustness through triangulation, discovery of new regularities, transformation of intervention practices, etc.). Ultimately, combining first and third-person descriptions is an actual example of a genuinely interdisciplinary practice.