It depends on what you want to do on the second server, if the second server shall host web, email and dns like the first one but shall be controlled from the first server, then install the second server as described in the perfect setup guide until you reach the step of the ispconfig installation. The last steps are:
1) Create a mysql root user on the old server which is able to login from IP address and hostname of the slave server, this is required for ispconfig installation on the slave. The step is also described in the ispconfig multiserver tutorials.
2) Now download and install ispconfig on the slave, when the installer asks you about installation mode, choose "expert" mode and the installer will ask you if the server shall join a existing ispconfig servers etup and ask for the mysql root login details of the master then.