To answer the original question,
1) Set up xmpp-server._tcp and xmpp-client._tcp DNS records for both domails. For details, search for "jabber federation dns srv records."
2) For adding buddies, "Zimbra (localhost)" is correct. Enter the full email address. This also works for gmail accounts and any other jabber server with published xmpp srv records.
You'd probably need to take care of #1 and #2 even if the domains were on the same server.