I think people typically use Reverse Proxies in this case. ForFor example you can set up any request for http://a.com to serve the content from http://b.com/a. This page describes it in detail:
http://a.com to serve the content from http://b.com/a.
You can read more about this on http://www.apachetutor.org/admin/reverseproxiesApachetutor Reverse Proxies