Co-ownership of purebred dogs has been in existence for decades. There are several reasons breeders find co-ownership agreements useful, but there are definitely positive and negative aspects when dealing with co-ownership.
When a dog is "Specialled" usually by a professional handler the dog will travel and live with its handler for extended periods of time. The costs can mount up quickly so owners will find other individuals involved within their breed and co-own the dog to help share the expenses.
Co-ownership is also very popular amongst breeders placing a female puppy on open registration for future showing and breeding. Many times the breeder will place a condition into the contract to receive a puppy back from one of the litters before signing full ownership over to the adoptive family. It is very helpful for a breeder when they find a good show home for one of their female offspring because it is very difficult for one breeder to keep and show all the show potential puppies themselves. When the puppy matures and hopefully acquires its championship title the breeder will hopefully receive a quality offspring back in return to carry the lines forward with. By remaining as co-owner the original breeder will be able to apply their kennel affix to the puppy they receive and will also be breeder on the registration paper allowing the original breeder to be able to show the puppy in bred-by classes thus continuing to move forward with their lines.
There is a BIG difference between co-owning a male versus co-owning a female. Both co-owners must sign all paperwork involved in the breeding of a female. But, a male can be used with only one signature so it is more difficult to enforce a co-ownership agreement as far as breeding goes on a male. Many co-ownership agreements are placed in order to protect an offspring from over breeding or from lack of knowledge on how to create the right pairing. For example: evaluating a planned breeding through health certifications of both the male and female and ancestries along with pedigree research and structural qualities involved in order to try to create a better offspring than the original two dogs you began with. Sometimes a novice breeder will not know the proper steps involved to create the best breeding possible. If a breeder knows they have a promising male and finds the right show home for that dog they may remain as co-owner in order to obtain unlimited stud service and/or continue to have their name on board as owner should the dog become good enough to be "specialled".
Co-ownership of dogs is a very common practice so if you run across it do not be alarmed, but educate yourself before you enter into any contract because the American Kennel Club (AKC) and the Cavalier King Charles Spaniel Club, USA (CKCSC, USA) does not endorse or promote co-ownerships because situations change and many times a dog's fate can lie in limbo if the owners are not in agreement about a situation. Make sure your contract spells out in detail the obligations of each owner and try to include as many unexpected situations in writing as possible.
Remember contracts are only as good as each person signing on the dotted line!