Speaking forthrightly, setting-up a backup MX server is outside the scope of ISPConfig, and not directly related.
That said, there's no reason that you can't configure your ISPConfig server to be the primary MX server, and any other computer of your choosing to be the backup MX server. The steps described in Falko's tutorial apply universally, provided that postfix is installed.
Out of curiosity, have you noted the comments at the bottom of the cited tutorial? Unless you're prepared for some serious SPAM-related headaches, there are other options to be considered, such as http://www.mxsave.com/
(I do not use or endorse mxsave in any way.) What's it worth to you to have your MX backup? Business accounts start at $6/mo... seems pretty reasonable.