BIND and postfix only on second server, how to?

I'm pretty sure I saw example how to configure ISPConfig for that but I cannot find one so my question.

I have main server with contos and ispconfig and now I'd like to setup another one as secondary DNS and backup postfix.
So I plan to install only two packages but I'm not sure what else is needed to install ispconfig and how to configure ispconfig to ignore non existing services.