I've co-owned dogs before & as a breeder co-owned with other people over the years.

1st co-ownership was a Standard Poodle from 8 weeks old. He was out of one of the top winning Poodle of all time. The breeder wanted over $10K for the puppies or co-ownership. I loved this dog dearly. I trained him not only in obedience but started him in SAR & confirmation too. He was kept is show condition & had to go to an experience groomer 3x a week while his puppy coat came out into his full adult coat. All this was a lot of time, $, & his grooming alone was a HUGE under taking. When he reach 1.5 years old & I successfully acquired his championship full filling the co-ownership. The breeder suggested he was such a nice dog he should be shown by a top handler. I thought it would be cool to go watch him in shows while being campaigned. The breeder other to pay 1/2 the cost of his show career. Long story short I NEVER saw my dog again, except on a national televise dog show on tv. But I did get my handler bill faithfully every month. When he retired for stud from show career the handler kept him, & assuming he breeder & handler split the stud fee because I never saw any money. I was offered a son out of my co-owned which the handler would show. I declined of coursed.

2nd co-ownership was a Great Dane with a close breeder friend that just needed help showing her puppies she was holding back.
Everything worked out fine the first 2 years then my pup failed some of her health clearances. The breeder had a fit that I wanted to spay her & not breed her. The breeder wanted to breed her then register her puppies under her sister's name since she passed all health clearances, but her sister didn't have as nice of confirmation. I thought my friend was a ethical breeder & cared about the breed. It was a slap in the face when I was already so upset my beloved dog's health clearances did not pass hips & heart. I went ahead & spayed her & the breeder went ahead & try to sue me. I won but our friendship ended. 

3rd co-ownership was with a Bulldog. The Bulldog puppy came to me from a very busy owner for doggie daycare & training.
She was dropped off one day then never picked up back up from the owner. I received a phone call from the breeder telling me I was her new co-owner-?! But I loved that little Bully & she became one of my demo dogs a long side my GSD's. Sadly she died of a massive heart attack at only 11 months old over night. The breeder was very nice & supportive. She offered me a replacement puppy which I declined since I never was prepared to add a 3rd dog to my home in the first place.

I've co-owned 4 dogs I've bred over the years. 2 were altered by their owners by 6-7 months old which obviously was upsetting to me, but what can you do? The dogs were in loving homes so that's what matter most. The other 2 dogs the owners never certified / titled the dogs as promised. Both owners tried but they either lacked the skill or resources to complete. I blamed more myself since they were novices & I shouldn't have expected them to meet such high promises they envisioned without trying to put in any effort beforehand to accomplish.

I think co-ownership can work only if there is a short window of co-ownership by the breeder. When I co-own a dog I only do so for 2 breeding for a bitch or two free stud services for a male. After which the dog's ownership is transferred directly to the co-owner in full.
During the co-owned time I pay all routine vet bills, health clearance, & breeding expense. The co-owner takes care of daily care & expense plus working / titling. If the dog is not breed worthy then ownership is transferred directly to co-owner plus they will get a replacement dog without any co-ownership. But I'm not saying it has to be spelled out like this just that is needs to be fair to both parties & only for a short amount of time. You should not owe the breeder any more than what the co-owned dog would be worth after titled at 2-3 years old. If the dog would be worth $4-$10K then = 2+ pick puppies / split 2 breedings from chosen stud, then co-ownership is completed. That would equal to the value of the dog if breeder had kept back & titled themselves. Why should you have to owe any more?

NEVER EVER agree to a long term co-ownership that only benefits the one party & your basically just renting the dog for life.
That kind of set up always lead to issues. Not unless your the type of person that likes to lease a car you have to pay monthly to lease & maintain. At the end of the lease you still have to dish out tons of $. But you get to drive a new expensive car through a lease you might not afford through financing. This would be the same as a long term co-ownership that just never ends.
If you prefer to own your car eventually & so you finance instead. After 3-5 years of car payments you now own your car.
This would be the same as a short term co-ownership. At the end you own the dog in full. If you want to alter the dog or breed the dog under your name with your choosen stud you should have the right after the co-ownership is completed, etc.