I successfully 2-boxed FFXIV for several weeks on my medium specs PC on Windows XP. After upgrading to Windows 7 the same process doesn't seem to work. Providing details on the XP solution below and hoping someone can provide insight into how to get it working for Windows 7.

For Windows XP:
1) You'll need 2 Windows XP user accounts. Create a 2nd one if you only have one.
2) Open Windows Explorer, open C:\Documents & Settings folder and browse into the "My Documents" folder for the user you installed FFXIV for. There should be a "My Games" folder with subfolder of "Square Enix -> Final Fantasy" etc etc (This is NOT the same folder that is in Program Files). Copy the entire "Square Enix" folder to the My Documents -> My Games folder within the new User you just created, so both users have the same folder structure.
***Note: If you copy this folder incorrectly, when you try to perform Step 4 below, the updater will open but you'll receive an error that "the update failed". This means you did not copy this folder to the right location and the program can't find the needed version information to run and get updates when running as the 2nd user. Double-check the folder location for the 2nd user and make sure it exactly matches the first user.
3) Start FFXIV as you would normally. Get past the updater, login, and click Play (don't open second instance until you get at least to the Square Enix login page, it won't work).
4) Tab out and right-click your FFXIV shortcut and select "Run As...". Choose the other user and click Ok. The updater will open, login, etc.
5) You now have 2 instances of FFXIV running on one computer!

Unfortunately when I follow this exact process in Windows 7, FFXIV is still able to detect the 1st instance of the program even though I'm running it as a different user, and it throws an error message when I try to start it up since the developers do not want you running multiple versions of the game.

Any ideas are very much appreciated. As a last resort I suppose I will dual-boot to XP just for playing FFXIV.

My computer specs when dual-boxing (no overclocking):
Q6600 (2.4ghz quadcore)
8800GTS 640 MB
Windows XP 32-bit