ISPConfig series 2 does not support multiple servers. On the moment ISPConfig 3 is under heavy developement. ISPConfig series 3, will support multiple servers.
I think there is no release date planned yet. As said, ISPConfig is under heavy developement. Yesterday Till, released a new Alpa version. Have a look here: http://www.howtoforge.com/forums/showthread.php?t=13851&highlight=released