On my quick review, I do not think this feature is available.
I also do not think this would be widely used.
This is more of a job allocation workflow issue than a server redundancy issue.
(I think? - I might have misunderstood)
The best option for you at the moment is to just have two systems (URLs) that customers can use one for internal issues and another for Master/Organisation Issues.
You could maybe use LDAP to connect these logins together in some way.
If I was going to action this workflow, I would have these systems separate. Let the low level issues be handled by customers Zammad system / IT team. Then allocate two or three customer users (IT Team I guess) to report bigger problems to the master ticket system.
I understand this might not be what you are looking for but hopefully this might have helped you in some way.
(Just a forum user)