Interoperation between permissioned consortium blockchain networks relies on their abilities to discover and validate the identities of each others' participant organizations. These organizations may possess self-sovereign decentralized identities and verifiable credentials issued by well-known certification authorities. Two mutually untrusting networks of organizations can establish a basis for interoperation if they have one or more certification authorities in common. Yet, for privacy reasons, neither of them may want to expose a priori their entire lists of authorities, necessitating a negotiation process through which common authorities can be identified. In this paper, we analyze this negotiation problem, and propose and analyze two solution approaches, one involving active participation of the trust anchors and the other without involving them.