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.

Some Multiboxing Tools, such as Mubox's "Game Isolation" feature, ease the task of symbolic linking your game files. Mubox supports symbolic linking on all versions of Windows starting with Windows 2000 (e.g. Windows 2000, XP, Vista, Server 2008, Windows 7, etc)