I would not co-own with someone unless I've known them for a very long time and we have a great relationship. It would also take a lot of planning and agreeing on conditions.
It is very easy to get screwed over in a co-own, I've heard several "horror" stories of breeders taking in the dogs on co-owns and breeding the heck out of them (to the point where you don't get to see the dog often). Not to mention disregarding the owner's input and breeding to other dogs that aren't breeding quality. There's just a lot of ways it can go bad.