Ok. To get sure i've checked the configs of our plesk server at work. For each domain i've enabled ssl i have a <VirtualHost <ip>:443> with a different "Servername <domain>:443" param. The ip is always the same and so is the path to the ssl cert file.
So it seems that you should trust this board and not the apache docu.
I would like to see support for this in ispconfig anyway. ;-)