[mcnallym]

You will need to register your DNS Server IP addresses with both ISP. ie each ISP knows about both IP addresses for the DNS Server.

If I remember DNS correctly (probably don't) then the ISP will forward the request to the first IP address, then if it doesn't get a response (as the ISP Link is down) it will request from the secondIP address.

If the DNS makes a request and the DNS on the just can' be resolved then you would get a response back that unable to resolve and it won't attempt to send to the second.