For purely cookie less static content, I system link 4 folders between 2 domains. I've done this countless of time using different host, different configurations, and yes, even different control panels when needed (and of course without). Noticed though that when making a symlink, that they're not exactly followable via ftp.
Does not matter if I make this using the user or even as root. The link is made (which is good), but not exactly followable.
ln -s /home/user/web/SITE-1/public_html/data /home/user/web/SITE-2/public_html/forums/data