We would like to do proxy pass to alias a URL 1 with URL 2 (with a virtual directory) but using the same content for URL 1

It's for performance and SEO reasons why we need to do this. Here is a better example of what we need to do, we have to sites www.msn.com and blog.msn.com that need to be on different servers because of performance (both of them have virtual servers and both of them are load balanced, so www.msn.com has 2 servers being used and blog.msn.com also has 2 different servers). We would like for blog.msn.com on the browser to be changed to www.msn.com/blog in the URI, but have the same web content for blog.msn.com. The reason for www.msn.com/blog is for SEO purposes where are searches in google and bing would be better.

Thanks