Hi,

>> I think you would need to use an additional box running something like
>> Squid as a reverse proxy to achieve this.

When it is possible to host both websites with one apache installation  
(on one server), then you could use name based virtual hosts:
http://httpd.apache.org/docs/2.2/vhosts/name-based.html

No additional proxy would be needed, and the webserver can  
differentiate requests to the two domains according to the  
HTTP-Request-Header.


HTH,
  Harry