I'm sorry but it doesn't work that way, I tried and ended up having sites flip flopping between the two servers.
what happens is all ns1, ns2, ns3, ns4 have the same value then they are all tested the fastest one is used and counters are set on the others which count down until they hit 0 then all are tested again. If you have two servers that are up then it could bounce ns1, ns4, etc and if they are different servers then you end up spreading your email over two boxes and in the case of webmail it gets very messy as if a client logs into one server he won't see the email on the other server and vice versa.
If your goal is just to have the website (static that doesn't update often) redundant then it would work. But a dynamic site such as a forum, or a site you use email this can cause havoc. I learned this the hard way and for 24-48 hours watched helplessly as it bounced back and forth.
I hope this helps someone for some really good info http://www.acmebw.com/askmrdns/
is a great link.