Mapping a secondary domain to a subdirectory using mod_rewrite

I am looking to map a secondary domain to a subfolder on my document root.

For example, if requests to the domain www.example.com map to my DocumentToot, then requests to www.exampletwo.com go to /sites/files/.

I am unable to accomplish a redirect from www.exampletwo.com/index.html to www.exampletwo.com/sites/files/index.html while making the URL still display www.exampletwo.com/index.html. Any ideas?

I believe you're looking for something like this:

RewriteCond %{HTTP_HOST} ^(www\.)?exampletwo\.com [NC]
RewriteRule ^/(.*) /sites/files/$1 [L]

RewriteCond %{HTTP_HOST}   ^(www\.)?example\.com [NC]
RewriteRule ^/(.*)         http://www.exampletwo.com/$1 [L,R]

RewriteCond %{HTTP_HOST}   ^(www\.)?exampletwo\.com [NC]
RewriteRule ^/(.*)         http://www.exampletwo.com/sites/files/$1 [L,P]

The P flag uses the proxy module, therefore the url is not changed (no redirect) on the client.