Me again :)
Are there any serious drawbacks to hosting both nameservers on a single server?
How much redundancy would I gain from hosting one on my server and one on a service such as granitecanyon.com?

Well if your named dies, its game over. With a secondary server you gain a little more protection.

It's a good idea to have another dns server on a separate backbone, that way if a route loop takes our your netblock, your other dns server would still be in action.

This might not buy you much, but for instance, on small outages, mail might transiting to your servers might sit around and retransmit until the connection is back up, but if there's no dns at all it will instantly bounce back to the sender.

I've had route loops happen before and having dns on different backbones did help a little (but its a far cry from redudancy). What might be good to is arrange a secondary MX record for the other server on another backbone to store the mail in such an event as well.