I don't have any experience with co-ownership, so this is just for my own understanding without any judgement to anyone that co-owns.
I can see where there may be an appeal to co-own a nice dog where both parties are active trialers. Costs can be shared. Both can handle the dog and enjoy it. Obviously any income can be split.
What I don't understand is a situation where there is a co-owner that may not actively handle the dog. I'm not familiar with the specifics, nor am I judging the parties or looking for the specifics of the business arrangements, but for example the co-ownership of the most recent NAFC. Looking at history, it seems only one of the two owners handles the dog. There were several people praising Mr. Kahn for the foresight to enter a co-ownership for the dog, and with good reason. I'm just curious why one might own part of a dog that they aren't running. Do the motivations for a partnership like that change between an active trialer and one that doesn't typically run their own dogs? Just a general question, not necessarily specific to that particular partnership.