Multiple addon domains, one htaccess file
RewriteCond %{HTTP_HOST} ^www\.addon1\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/show.php
RewriteRule ^(.*)$ /show.php?domain=addon1.com&$1
RewriteCond %{HTTP_HOST} ^www\.addon2\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/show.php
RewriteRule ^(.*)$ /show.php?domain=addon2.com&$1