Here are some questions:
Normally, the Primary entry is searched first. I assume that if the domain name in my URL isn't found there, the Secondary address is checked automatically? If I had a 3rd entry, it would check there if the Secondary failed to return a result?
If a DNS resolve failed (NX Domain), nothing will happen, except your client can not resolve that name. Only if the connection to the dns-server fails, the 2nd, 3rd etc. is used.
-> Redundancy of DNS is normally done on the client, no on the server (just by the way some people think of using 1 HA DNS-Server instead of 2 or more on the serverside...)