Hello, I have a website hosted on a dedicated server. But sometimes server goes down and I have a backup shared hosting package that I want to use.
I want to use following nameservers:

NS1.DEDICATED.COM
NS2.DEDICATED.COM
NS1.SHARED.COM
NS2.SHARED.COM

How will it work? I don't want visitors to go on shared hosting when dedicated server is working. I want them to go there only when dedicated one is down. Is there any alternative way?