I've been working on something similar. You'll have to decide if your slave server is going to be an additional server in your setup, controlled by your one ISPconfig3 panel with it's own websites, email domains and sql or will it be an mirror or your master server.
If it's going to be a mirror you again have to make some more decisions. Setting it up as a mirror is fairly simple. Take the expert set up, I'd advise giving it the same route password as the master and then on the master make a new mysql root user, but with the slave's host details and IP instead. This will then give you two servers wih the same configuration (same vhost files etc). But then content is handled separately, and when I say content I mean /var/www/ and databases etc. I've used rsync top copy content. I'm also using master slave database replication in mysql to copy certain databases across. There are other methods such as shared file systems on a SAN for example which is where both servers could access the content from, but for me that mean there would be another single point of failure... the SAN.