In case it helps anyone, for the second DNS server we use PowerDNS. I use this _basic_ script to add the entries to the PowerDNS MySQL database. I assume you can figure out how to configure PowerDNS and MySQL on the other server yourself. It should be connectable from plesk.
Within plesk I use event manager to run it. Like so:
Domain alias created lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliascreate
Domain alias deleted lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliasdelete
Domain alias updated lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliasupdate
Domain created lowest (0) nobody /usr/local/scripts/addSecondDNS.php domcreate
Domain deleted lowest (0) nobody /usr/local/scripts/addSecondDNS.php domdelete
Domain updated lowest (0) nobody /usr/local/scripts/addSecondDNS.php domupdate
Within plesk I use event manager to run it. Like so:
Domain alias created lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliascreate
Domain alias deleted lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliasdelete
Domain alias updated lowest (0) nobody /usr/local/scripts/addSecondDNS.php domaliasupdate
Domain created lowest (0) nobody /usr/local/scripts/addSecondDNS.php domcreate
Domain deleted lowest (0) nobody /usr/local/scripts/addSecondDNS.php domdelete
Domain updated lowest (0) nobody /usr/local/scripts/addSecondDNS.php domupdate