Site Access - Domain 2 to Domain 1

From thread: http://tinyurl.com/38hkxv

Just as an addendum to this topic, you can share across hosted domains in a similar way as described above.
http://sites.google.com/a/domainname1.com/e/domainname2.com/SiteName/Home
In this URL, note that domainname1.com is the site that the user is logged in to.
domainname2.com is the hosted domain that you wish to give them access to, with the Google Site Name (SiteName) appended to the URL. I think '/Home' is optional.