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.