NOTE: This answer is about DNS client configuration. After some comment discussion, it now appears to me that the OP could be asking about DNS server or domain DNS configuration. If that is the case the premise holds (NS2 only hit if NS1 not available), but the specifics are not relevant.
The DNS client queries the secondary DNS Server only when the primary DNS server does not respond. If the primary responds with "sorry, wrong number", that response is passed back from the DNS client to the application attempting to communicate.
While a particular DNS client can be programmed to do whatever it wants, the standard is to go in order.
From the Microsoft Technet Understanding DNS client settings page - emphasis mine.
Configuring a DNS servers list
For DNS clients to operate
effectively, a prioritized list of
DNS name servers must be configured
for each computer to use when it
processes queries and resolves DNS
names. In most cases, the client
computer contacts and uses its
preferred DNS server, which is the first DNS server on its locally
configured list. Listed alternate DNS
servers are contacted and used when
the preferred server is not
available. For this reason, it is
important that the preferred DNS
server be appropriate for continuous
client use under normal conditions.
From the Ubuntu Resolver man page - emphasis mine
The different configuration options
nameserver Name server IP address
Internet address (in dot notation) of a name server that the resolver
should query. Up to MAXNS (currently 3, see )
name servers may be listed, one per keyword. If there are multiple servers,
the resolver library queries them in the order listed. If no nameserver
entries are present, the default is to use the name server on the local machine.
(The algorithm used is to try a name server, and if the query times out,
try the next, until out of name servers, then repeat trying all the
name servers until a maximum number of retries are made.)