You could just go ahead and create the subdomain, then upload your site to it. Once you do that and make sure everything is working okay, you could do either of these:
Redirect your other domain to your new sub-domain.
Mirror the subdomain with the other domain name.
If you're worried about stuff like duplicate content penalties, I'd stick with a redirect.
Save up to $96 at Dreamhost with ALMOST97 promo code (I get $1).
Or save $97 with THEFULL97.