This is now working for me on the 4 different sites that I have.
You have to be sure to put in the exact path of the forum - not just the site name. If your forums, like mine, are in
http://www.yoursite.com/forum and
http://www.yoursite2.com/forum, you need to enter the following into the box:
www.yoursite.com/forum
www.yoursite2.com/forum
Omitting the /forum part or including the http:// breaks this.
Basically what's going on here is opening hidden windows and posting the form information into them. As long as user names and passwords are the same, your good to go.