[thelist] .htaccess for two URLs

[Fred Jonest]

I have two domain names and I want the secondary one to be sent to a 
subdirectory, meaning that if you request 
http://secondsite.com/aboutus.php the server will deliver 
http://mainsite.com/secondsite/aboutus.php

I tried this

RewriteCond %{HTTP_HOST} !mainsite.com
RewriteRule ^(.*)$ /secondsite/$1

but it fails.

Any better ideas?

Thanks,
Fred