This just an artifact of english usage. Joe and Jim start a company making widgetry for social networks. Joe is a founder. Jim is a founder. Joe and Jim are t

This just an artifact of english usage. Joe and Jim start a company making widgetry for social networks. Joe is a founder. Jim is a founder. Joe and Jim are the founders of the social network widgetry company. Jim is Joe's co-founder. Joe is Jim's co-founder. The "co-" prefix should really only be used where it removes confusion and makes it clear that there is more than one founder.