There are many ways of creating a contract to meet the needs of each partner when a buck or any animal for the matter is co-owned. The number one and above all most important thing in doing this is that ALL parties clearly understand the WRITTEN contract. Yes a WRITTEN contract is key to a successful partnership in ANY animal.
Contracts can and should be drawn up to cover who houses the animal, feed costs, med costs, testing costs, is there insurance to be carried on the animal and if so who pays for it, number of semen collections each year, what time frame between collections, who covers the cost of the collections, where will semen be stored, who is in charge of the inventory for the semen, method of price determination of that semen, how will semen be marketed such as will it be offered at auctions and how many straws per year to be sold, what happens with the farm use only straws, what will the policy be for straws that have had issues to those who purchased them, who gets what number of live breedings each season, will live breedings be sold to others, can a partners live breedings be sold for their own profit excluding their partner, how will the partnership be closed out at a future date, antler sawing or not sawing, antler replica sales, and the list goes on and on of items that should be addressed prior to partnering with anyone on an animal.
What seems like small details can later become sore points in a partnership that leads to hard feelings in the end.
Looking back during my time in the industry and the partnerships I have been involved in and the partnerships I have heard others talk about, I would say there should not be a 50%-50% partnership on any animal. One person should always be the controlling factor in the partnership. There are simply too many variables to plan ahead for each one of them. As these variables pop up if both parties or all parties always agree on things then all is good. However when they don't all agree all can go to heck fast. I also think that selling or buying into a partnership via auctions is not a desirable way of doing business. There simply are too many things to be considered in a partnership for it to be auctioned off to whoever is in the room. Who you are partnered with is as important as what your partnered on or the investment/profit of becoming that partner.
Just some of my ramblings about my opinions on the topic.