not possible multiple primary dns server?
 

it is not possible for a zone to contain multiple primary dns server right? it can only contain 1 primary and multiple secondary?

No, there is no such thing as "multiple secondary" either. A 3rd DNS server would be a Tertiary server and so on. :) In the world of DNS servers, someone has to come 1st, someone else 2nd, someone else 3rd, and so on. The primary server is merely the first server contacted, if it's unavailable then the 2nd one is contacted, if it too is unavailable then the 3rd and so on, if they exist.