Hi, you should be able to do this through a proxy RewriteRule. When you said you’d tried that and it didn’t work, did you mean the site appeared broken or that you couldn’t get the RewriteRule to work? If the issue was that the site appeared broken, keep in mind that “b.com” in your example needs to send back HTML to the browser that is completely valid, including all URLs being valid with how it references them. If the HTML sent back when you access “b.com” uses URLs starting with “/” that use paths that only exist on “a.com”, that will result in broken URLs.

The proxy approach works great (I’ve used it before) but it does exactly what you tell it and browsers will continue to work exactly as they do. The browser doesn’t know there’s any relationship between “a.com” and “b.com”, so if the HTML on “b.com” has paths like “/img/foo.png” that actually only exist on “a.com”, you’ll get 404’s.—Of course, you may be able to solve that with more RewriteConds/RewriteRules (e.g. check if any requested file exists, if not, proxy through to a.com).