Unbound: Multiple Servers for Domain Override

[GrueneNeun]

If i enter multiple servers for the same Domain and the first server is offline, a client requesting DNS entries for the overridden domain is getting an error. Is it possible to have multiple entries for the same domain, so that even if one of the servers fails, a client is getting correct results?
I am trying to use the firewall as main DNS server for the clients, but since we are using Active Directory, our internal domain must be available too, even if one of the Domain Controllers is offline or updating or something like that.