having the exact HTML (including title and metas) for both sites could get you penilized by some search engines (looks like spam).
having both sites with slightly different titles could give you multipull listing in the SE's. It may not double your hits, but it should increse them.....
one problem though...competitors. if they see you have 2 sites that are basicly the same they could report you to the SE's in an attempt to get you banned or penilized.
the other idea may be to mirror your new site with ther old and replace all your old url pages with auto redirects. Justr register the new site. as the engines reindex your old site the pages will be removed from their index....this way people that know your old site will still be able to find it.
or not :o)