I don't think I would do it this way, RWesley.
Why do you need both versions to be live and indexable at the same time? Most of the time people use a development environment for this type of thing.
I guess I'm a little confused about the strategy here. Could you explain more about the reason you don't just do all of the work on a non-indexable, private development site and then push it out live to replace the existing site on www. whenever it is complete, along with the redirects from old content URLs to the new ones (if that's even necessary; which it is not if you're replacing old content on the same URL).