As it stands now you cannot have a resource with the same id linked to multiple users. This causes an issue if you have a multi-site setup and a user has distinct accounts across multiple sites (in this example case each site is distinct and users are not shared across sites).
Adding an optional siteaccess field to OAuthEz and the ability to lookup in the repository by ResourceUserId and siteaccess should allow this.
I see also that the security user is looked up by email. Maybe that rules out the ability to have multiple users with the same email address?
What do you think?