Gilas, if you dont believe me, read the apache documentation.
If you use ISPConfig or not is up to you, so telling me that you will change to another control panel is up to you too and you will not get more or less help if you tell us that.
Pleask may use a patched apache server that supports more then one SSL Cert per IP, but the apache servers that were delivered with the linux distributions definately do not support it.
Please search the forums, you will find several thread for this.
Additionally, have a look here: