The IPB folks might have some better tips, but it might be worth toying with mod_rewrite for a bit. (if your regex skills are ok) Mod_rewrite can be quite a beast and it usually takes me a bit to get it working, but here's a quick example:
(put this in a .htaccess file in the same directory your index.php file is for IPB)
Code:
RewriteEngine on
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{HTTP_HOST} forums.crossmediagroup.net(.*)
RewriteRule (.*) http://business.crossmediagroup.net/forums/index.php?%1 [R,L]
So for each hit to forums.crossmediagroup.net they'll be bumped to the other subdomain. In my quick tests I couldn't find a way to do it without using the
R modifier (redirect), but it's worth a try.
You might also just want to make two copies of your IPB installation and try changing some of the values in the config file.
-r