To display another field in specific form only you need to add context to xml of the Many2One field Then you will override name_get method in the model of Many2one field as below 

def name_get(self):
if context is None:
    context = {}
res = []
if context.get('special_display_name', False):
    for record in self:
        res.append(record.id, rec.YOurFieldYouWantToShow)
else:
    for record in self:
        res.append(record.id, record.name)
return res

please check the below links for 

https://stackoverflow.com/questions/31724556/odoo-display-name-of-many2one-field-combination-of-2-fields