With a female the breeder has more say, but with a male each owner is individual so trust becomes are large aspect of co-ownership agreements also.
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. When problems do arise the dog's future can remain in limbo. Make sure your contract spells out in detain the obligations of each owner and try to include as many unexpected situations in writing as possible. Contracts are only as good as each person signing on the dotted line!