One Identity for All Services
When accounting for partner organizations, like any user account/identity relationship, we recommend a singular view of a users accesses. Meaning, we do not endorse users having multiple accounts for multiple services, because the overhead and risk outweighs the benefit of architecting solutions that follow the One Identity Principle
Your users may shop, pay bills, renew contracts, and more using one unique user identity, only allowed to do those things across multiple sites if they should be able to. Backed by strong RBAC/ABAC, multi-site SSO is the best way to allow users the convenience of SSO across different services using strong industry protocols like Oauth and SAML to federate Identities.