Generally, ISPConfig configures the services like apache, bind postfix etc. trough their config files. If you want to mirror ISPConfig, you will have to mirror these config files and the ISPConfig mySQL database too. The config files are:
Vhost_ispconfig.conf (in /etc/apache/vhosts or similar)
/etc/passwd, /etc/shadow, /etc/group /etc/gshadow
named.conf and all the include files.
problematic might be the system quotas too.
I dont know what happened to the apache logfiles for ISPConfig, written in /var/log/httpd/ and the proftpd xfer.log
If you switch to the second server, any service must take care that the services on this server has loaded / reloaded their config files.