The term "co-grandfather" refers to two individuals who share the role of being a grandfather to a child. However, there are several synonyms that can be used to describe this relationship. One commonly used term is "joint grandfather," which emphasizes the shared nature of the role. Other options include "co-grandparent," "shared grandfather," "two grandfathers," and "double grandfather." These terms can be used interchangeably depending on the context and the preferences of the individuals involved. Ultimately, the most important thing is to have a loving and supportive relationship with the child and to cherish the special bond that comes with being a grandparent.