In most cases the secondary server is never going to be queried for any lookups, internal or external. Assuming that all of your DNS clients have the primary server listed as their primary DNS server and have the secondary server listed as their secondary DNS server, the only reason a DNS client would ever query the secondary is if the primary server were unavailable. If that in fact describes the problem you're dealing with, then I think you're addressing it from the wrong direction. If you are having problems with the stability and reliability of the primary server then my suggestion would be to fix those issues. This is of course based on the assumption that your set up is fairly basic; one site (office, location, etc), 2 DNS servers (primary and secondary), and that all of your DNS clients are configured to use the primary server as their primary DNS server and the secondary server as their secondary DNS server.
That being said, the secondary should be capable of resolving external queries. There are scenarios where the secondary may come in to play:
The primary server is unavailable
The secondary server is set as the primary DNS server for some DNS clients.