I know some people have created two accounts one for the front page, and the other for the forums... and the usernames might be totally different.
fileplanet, gamespy, network had similar problems during the merge but managed to merge all accounts into one.
I know the sites are in different domain or sub-domain but am sure there's a way for the forums to detect that you were logged into the other front page site.
Otherwise if you log into the front page site, a background AJAX request shoots off to log you into the forums... and vice versa..
Ofcourse first to merge or "link" multiple accounts....