Every school and organization makes up its own rules for promotion. Who should sign a certificate is up to them, it doesn't even have to make any sense. What makes sense to me, is that the most senior instructor responsible for promoting people should sign the certificates. If there is no one who is your senior in your organization, then there is no more promotion...you're the boss. If the promotion doesn't come from a teacher who is your senior, then to me it wouldn't mean anything...but this type of rank/grading stuff doesn't mean anything to me anyways.
The Japanese Dai Nippon Butotukai is an example of an organization which oversaw more than one style. While not all coming from the same style of budo, the masters on the board would recognize the achievements and skill of other martial artists, and award them with titles like renshi, kyoshi, and hanshi.
Ranks and grades aren't real, they are onnly relative to the organization that awards them. What happens is, a group of advanced students will break away from their first organization to do things their own way. They appoint one of their number as the head of the group, and give him or her a new title, and from that frame of reference begin a new rank hierarchy. So dude who was a fifth degree in Karate Org USA is now Hanshi, 10th degree of All American Karate Org, and the guys who who went with him, who used to be fourth and third degree are now eighth and seventh degree. They start out with a new set of schools and new white belts who start climbing the ranks, never the wiser. Eventually some of their students will do the same thing, after finding out that their "hanshi" only ever actually achieved 5th degree from karate org USA. Meanwhile new students continue paying for rank advancements and new belt colors and organization patches, until they too see through the illusion of "rank".
The important thing is skill, time and effort, and the ability to teach those skills and inspire others.