I know this sounds strange, why would you want to use 2 different themes. Here is one reason..
There is a plugin I can use for reviews, each commenter can vote and review a product. The problem with this plugin is it’s hard coded into your theme, this means that every post you make has the voting results on it. If you only want a review/vote post once a week, what are you to do?
This is what I ran into when testing the review plugin for another site, my answer? HeadSpace2.
HeadSpace2 is a plugin to manage meta data for your WordPress site. For the unfamiliar, meta data is all the extra information that explains what your content is about, allowing you to fine-tune your site for search engines. HeadSpace2 tries to make the management of this data as easy as possible so you can spend more time creating content.
Headspace2 not only lets you control metadata but also the theme you use for each post. For my problem though, I wanted it to look like I was using the same theme, just not the plugin all the time. So I created a copy of the theme (by changing the data at the top of the CSS file you can rename it) uploaded it to my site and now when I create a post that I want commenter’s to vote on, I tell HeadSpace2 to use theme#2 for the post. Since everything else looks the same, it looks like I just turned on an option for the voting.