Only one secondary name server can be entered in the Zone Information dialog, but MacDNS does support multiple secondary NS records. To specify more than one secondary name server, you'll need to edit the zone file manually with a text editor.
Look for the line that says:
; Nameserver entries
After this line, you'll notice the secondary name server (NS) entry. If you need more secondary servers, just add additional lines.
For example:
mydomain.com. NS secondary.provider.net.
mydomain.com. NS another.provider.net.
mydomain.com. NS third.provider.net.
mydomain.com. NS fourth.provider.net.