
I own a dog jointly and we have all signed some white labels and we use these, every so often I send new labels for signatures. When registering litters sired by him we send the registration form to each person in turn. I am sure that any person checking could tell that the signatures were not by different people. don't think it seems to matter now so much with show entries as they can even be done online, and they don't get a signature then.
There was a recent thread on this subject with some things to think about and agree in writing before entering such an arrangement.
some things to think about are who pays for what? Will you allow the breeder to use the dog at stud, who gets the stud fee? How long does the agreement last for? who pays the entry fees, gets prizes, and a whole lot more.