Hey,
This is, for as far as i can see, practically impossible because you will always have one authoritive DNS server for your specific domain.. You cannot ask a different ISP to admin your domain.. The only thing you can do is have for example site1.com hosted with 1 providerpoiting to you and then create a second site which is site2.com and let it point to site1.com.
Also, if you have a good ISP they will have multiple DNS servers, ask them if your not sure. They will always have some sort of recovery model (or disaster recovery) for this kind of issues.. If their DNS servers fails they will ahve a small issue :) :)