It depends on the scenario. With on DNS for your intranet, a domain controller for example, you don't need two. Just tell your internal DNS to forward all external addresses to the next DNS: http://en.wikipedia.org/wiki/DNS_zone
Internal network you can have as many DNS servers as you want. In your case, two is preferred for internal resolution. Each DNS server should be setup with DNS Forwarding to an IPS DNS if you want web browsing.
As mentioned, DHCP should hand out both DNS server IPs (or more) and you set your preferred (first, second, third, etc) DNS server order.