co-branding
When brands agree to cross-promote each other. In effect, each brand becomes a separate sales channel for all the others in the group.
When a driver of a high end sedan gets out of the car wearing an expensive watch and a bespoke suit, now that is co-branding. Each of the three brands have agreed to promote each other in RL (Real Life), on television, in print, online, in social media and on the mobile web.