I think you mean. I've got apache answering for x.y.com and its document root is set to, say: /var/www/htdocs/index.html, meanwhile I've also got aa.vv.com aimed at that machine and I want apache to know its document root is: /var/www/htdocs/aavv/index.html
Is that right?
If so, Virtual Domains.... take a look at your httpd.conf file and page down to near the bottom. Remember you'll have to create a virtual entry for your primary domain too.
Also, this is so NOT a hardware question, I'm gonna move this one to Software.