RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteCond %{HTTP_HOST} main.subdomain.com
RewriteRule ^(.*)$ index.php?$1 [L,QSA]

RewriteCond %{HTTP_HOST} other.subdomain.com
RewriteRule ^(.*)$ index.php?other=$1 [L,QSA]