Thanks for that till.
So I just add the 2nd server in as a secondary into the ISPConfig web, and create a slave on the second, or is it a full record?
The problem I was having, was specifying a primary and secondary via the portal, which propagated the record to the second, but would not update the record on the second when changes were made.
Am I able to do this via a database replication to the other server instead, as I would prefer to only have one site to log into (others will be doing this, and am trying to make it as simple as possible). Such as cron job to the other server?