When selecting secondary servers, attention should be given to the various likely failure modes. Servers should be placed so that it is likely that at least one server will be available to all significant parts of the Internet, for any likely failure.
Consequently, placing all servers at the local site, while easy to arrange, and easy to manage, is not a good policy. Should a single link fail, or there be a site, or perhaps even building, or room, power failure, such a configuration can lead to all servers being disconnected from the Internet.
Secondary servers must be placed at both topologically and geographically dispersed locations on the Internet, to minimise the likelihood of a single failure disabling all of them.
That is, secondary servers should be at geographically distant locations, so it is unlikely that events like power loss, etc, will disrupt all of them simultaneously. They should also be connected to the net via quite diverse paths. This means that the failure of any one link, or of routing within some segment of the network (such as a service provider) will not make all of the servers unreachable.
It's a best practice to split primary and secondary DNS servers to independent hosting providers to avoid be in the situation like this
You can get more info at http://tools.ietf.org/html/rfc2182