Cookies only work with one URL, nothing you can do about it.
I suggest you make a perm redirect from
www.centralrock.info to
www.tv-digital.info/board, BEFORE the forum is loaded. You could do this with a mod-rewrite in .htaccess:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.centralrock.info$
RewriteRule ^(.*)$ http://www.centralrock.info/board/$1 [R=permanent,L]