The setup that you used above is for dedicated systems where each server hosts different services, so the setup you had choosen is not a mirror setup, it is a setup that splits email, web and dns to different servers.
A mirror setup is e.g. this one, http://www.howtoforge.com/installing...th-ispconfig-3
which allows you to access sites on both servers simultaniously without the need to switch any IP addresses. If you want to switch IP's between servers then take a look at e.g. keepalived or heartbet which can be used together with ispconfig 3.