Multiple servers in ispconfig normally does not mean mirroring as ispconfig is solution for ispc's to manage a large farm of servers that run different services, e.g. 5 mailservers, 20 webservers, 2 dns servers and 5 database servers from one controlpanel. So a non mirrored multiserver system can be setup step by step when you need additional resources as every server is treated independantly. If you want to setup a mirror and not a multiserver system, then the mirror has to be setup before you add any data as both servers have to be in sync and you have to use special filesystems for /var/www and /var/vmail to replicate the files too.
ISPConfig requires it that a mirror is configured before any data is added.
In your case, it seems that you did not use a network filesystem or SAN for the data drives yet, so you will have to setup this first and then you can try to copy all config files from the master to the slave. The only clean solution is a reinstall were you take care of mirroring from the beginning.