Another Rewrite Question

[penagate]

You don't need two licences to do that if your other domain name encompasses the subcategory.

RewriteEngine On
RewriteCond %{HTTP_HOST} ^example\.com$
RewriteRule ^$ forumdisplay.php?f=X [L,QSA]

Other URLs will work with no changes as long as your domain is correctly pointed to the server. The rewrite is only needed to change the home page on your second domain name to the forumdisplay page of that subcategory.