My multisite installation works great using subdomains and domain mapping for top-level domains. But now I want to make a URL under the domain of one site be the root of another site. That is, I have http://<site1> and I want to make it so that if I visit the page http://<site1>/<site2> I'm looking at the front page of another site. Is that possible strictly inside WordPress, or do I need (e.g.) an Apache ProxyPass directive?