What would be happen if one of two dns was down?

[Mark Boolootian]

> > But  when ns1.example.com dns was down, actually some people can't  
> > lookup the domain.

What Chris said.  If a user's system (stub resolver) is only
configured to know about ns1.example.com, when ns1 becomes
non-responsive, the net breaks for said user.  The user should have 
both nameservers configured for their stub resolver (e.g. /etc/resolv.conf 
should contain addresses of both name servers).