dns dual server question

I have two machines running on two different ips and have them both registered as ns1.mydomain.com and ns2.mydomain.com

My question is why if I take one machine down does it take so long for the dns to resolve and go to the second ns2.mydomain.com server? They do both work I can take either the ns1 machine down or the ns2 machine down and eventually the name resolves again and serves off what ever machine is up, is there any thing you can change to get them to resolve faster some times it takes 30 min or more.

To resolve what?

If I take down one machine that is currently say the ns1.mydomain.com why does it take so long for the second machine ns2.mydomain.com to start serving my web site, it will eventually take over and any request do go to the ns2 machine

You mean ns2.mydomain.com is your web server?

ns1.barnzee.com and ns2.barnzee.com are my name servers they are pointed at two different ip addresses. Right now this instance ns2.barnzee.com is up and serving the site, I have ns1.barnzee.com off line at this moment. Normally I have them both running at same time, however if ns1 went down it takes sometime before the ns2 machine starts serving the site, that is what my question was is there anyway to speed up the transfer so the second ns2 starts serving faster if the ns1 goes down.