Thing is if you got W7 with several accounts you can make startup scripts handle folders for you, hence auto rename interface and wtf between users. Only need to have the game installed anywhere but program files.
A .vbs script could do the trick. Let's say you are user1 and your friend is user2. When you login the script checks the \Interface folder for "user1.un", if found the \Interface folder is your folder, if not it checks for what the .un file is named then renames \Interface to \Interface.user2 and the \WTF.user2 respectively. Then it renames \Interface.user1 to \Interface hence now the game will load your own settings.
Each logon it must check for the .un file in order to distinguish who is who. I mean it's an idea, it would work.
I suck at .vbs but I wrote an example autoit3 script:
http://pastey.net/149990