I think you need to start your own thread instead of posting an answer like this. People tends to missed out because your question is not the primary one. You can think of reference is like many2one field but with many models (not one). The relationship is stored as : model_name,instance_id like: account_account,123. Not sure what this has to do with the main question. Now, if you have a many2many between X and Y, you can define a field in BOTH model using the SAME relationship table with the column name flipped. The relationship made in model X will automatically reflected in model Y.

Thank you Ivan, for your answer. Nevertheless I prefer staying on this topic as I don't want to create a second topic with the same question that Roberto. However, if you think I need to create another topic, I will do so. When I created a many2many type field named x_m2m in the X model, a relation have been automatically created in ir.model.relation with the name x_Y_X_x_m2m_rel. Then I created a many2many type field into the Y model named x_m2m. Another field have been created in ir.model.relation, x_Y_X_x_m2m_rel, which is the sale with column name flipped. Finaly, when I manage X and Y views to get x_m2m fields displayed, it doesn't seems to work : if I add an X instance in x_m2m field from the view of an instance of Y, the reciprocity is not found in the corresponding instance of X and its x_m2m field.