In the first half of the twentieth century, two thinkers with a strong presence in French-speaking psychoeducational circles, Jean Piaget and Roger Cousinet, developed theories that overlapped in a number of ways. Some authors have emphasized the proximity of their ideas, but this proximity conceals a much deeper dynamic. In fact, little is known about their relationship, and references to it are scattered. In this article, a microhistorical approach reveals the different configurations of the early 1920s regarding the nature of their relationship and the mutual influences between the two authors. In particular, Piaget's ideas were very close to those of the New Education movement, of which Cousinet was a central figure. Their relational and intellectual strategies were in harmony. This is the sign of a double synergy, both internationally and between disciplines: Piaget inspired Cousinet psychologically, while Cousinet stimulated Piaget pedagogically.