Originally Posted by Thurallor
I would suggest running the two client instances as different Windows users. Presumably that should cause the data files to be saved in the Documents folder for each user, although I haven't tried it.
If you shift+right-click the shortcut, you can choose "Run as different user", then be prompted for the username and password. I think you can also modify the shortcut so it will run as a different user when you click it normally, and you don't have to enter the username/password. Once again, I haven't tried it.
That's an interesting idea. Note, you would have to also install all of the plugins (and skins, etc) that you use under that other user's documents folder since if you are running the game as a different user, the Plugins folder will also be different. Requires a little bit of prep work, but it should work. However, any plugins that you DO want to share information will no longer share information (can't have it both ways).