A merger with consolidation of control into one entity/hierarchy is not intuitively what I consider to be a true partnership. From my point of view true partnerships are characterized by being build on and expressing interdependence, cp. Interdependence theory - Wikipedia.
The third listed option in my reply to Alex seems to correspond to some characteristics of interdependence.
The above Wikipedia-page focuses on group-dynamics. The concept of interdependence also has been applied to individual development, see eg. the books by SR Covey, eg. The 7 Habits of Highly Effective People - Wikipedia → providing a link to 9. Workforce capacity development and inclusivity.