another way, is to use custom templates. Have a look at subdomain.conf and the virtual_host* templates, where you can define your own DocumentRoot etc to your own liking. (subdomain.conf I had on an old server, maybe not used anymore)
Setting up simply as a 'separate domain' is probably much faster though!
( I was always worried that Google et al, would find my stuff at yy.com/xx and mark
as a duplicate of xx.yy.com, and penalise, but I could be totally wrong on that )