[keycloak-user] Authenticate against multiple realm management clients simultaneously

Hi, I use Keycloak to secure an application that has two types of users: vendors and customers. I created one 'customer-realm' and one 'vendor-realm'. Each realm also has one client which the application authenticates against, depending on whether the vendor login or the customer login is used. I also have a backoffice application that is separate from my main application. Backoffice users should be able to manage both vendors and customers. I planned to utilize the auto-created realm management clients in the master realm called 'customer-realm-realm' and 'vendor-realm-realm' that would allow me to assign permissions to users in the master realm to manage the other realms as needed. However, when a user logs in to the backoffice application, it can only authenticate against one of the realm management clients and not both (as far as I see). So the user would only receive half of the required permissions. What is the best approach here? Thank you!