You can only have one Starling Console per forest. This is because the settings are stored in Q.A.C. and One Identity recommendation is to put it into the top level of any forest.
That way it would be a single join.
If you want to add additional consoles, then you must add them in a separate domain or forest.
If you attempt to use the same license in both consoles, then it would generate a conflict because it is already used on a first console.
In order to avoid conflicts, each domain or forest should have its own unique instance of Starling Control Center license.
Your Account Representative can assist with the purchase of additional QAS licenses, and additional Starling subscriptions (if required).
In order to join an additional Control Center, being a Primary Administrator may not be enough. Some additional permissions may be needed.
The person(s) that holds the role of Organization Owner in Starling 2FA should grant the additional permissions to the owners of the additional control centers before they join.
The additional permission for Hybrid Subscriptions (purchased as part of another product): Hybrid collaborator
And can be accessed by the Organization Owner to create the invitation here: https://account.cloud.oneidentity.com/hybrid
It is also useful if the Organization Owner can promote the account to Organization Admin
Organization Admins can be invited/created by an Organization Owner here: https://account.cloud.oneidentity.com/settings/organization-admins