Hi Manuel
I don't think isolation mode is an option as this is applied to the whole site and prevents sharing of courses between tenants.
You can make a user a tenant member and this will restrict the user to resources for that tenant
You can also make a user a tenant participant for more than one tenant that have resources that you want the user to see.
This can be combined with tenant based audiences to only make visible some courses available to users is specified audiences.
Have a try of this and get back to us with any problems that you see with this.
Regards