If all you want is to *run* two instances, then it's much, much easier.

"Applications" on a mac are actually directories. When you double-click on them, it runs a program called "open" on the directory, which, amongst other things, checks to see whether the application is already running. If it is, it switches to it instead of running another instance.

To bypass this, you can just run the program directly. For World of Warcraft on my Mac, run this at a terminal prompt:

/Applications/World\ of\ Warcraft/World\ of\ Warcraft.app/Contents/MacOS/World\ of\ Warcraft

Under a unix, the "/" character separates directories and the "\" is the escape which treats the very next character as a literal; basically, it allows you to talk about files which have spaces in their names, since "World of Warcraft" is very different from "World" "of" "Warcraft". If you're not familiar with unix or dos, just ignore the explanation and run the line above blindly (with the caveat that I'm assuming you installed WoW in the default location, like I did).