From Dual-Boxing Wiki

Introduction

This page is intended for people that want to multibox using Windows Vista. It covers topics that provide Vista-specific solutions.

Symbolic linking

Symbolic linking is like placing a shortcut, but more 'deeply'. You can work with the link of a folder as if such folder is actually embedded in the root folder where the link is placed. This way you can have 1 folder with your WoW data and link to it from all your clients' WoW root folders.
In practice you could be working in c:\folder, while you're actually working in d:\otherlocation, because the 'folder' entry links to d:\otherlocation.

This page shows how to set up symbolic links:
http://en.wikipedia.org/wiki/NTFS_symbolic_link
This is a link to a topic that provides a powershell script that creates symbolic links for WoW: http://www.dual-boxing.com/forums/index.php?page=Thread&threadID=4854
Symbolics links are useful for WoW folders like "Data", "Interface" and more.