Building enterprise web sites from various graphic models involves two different level of customisation within the wordpress framework.
Page and post templates in wordpress
Themes are pieces of code aggregating various page types : archive pages, single post display, single page display. Most commonly all models of a theme share the same header and footer, and depending on specific coding operations, the same menu from the wordpress menu system. The theming system allows a lot customization options, including the possibility to define page templates for letting editors choose, for example, between full size page and 2 column page with widget enabled margin.
In native wordpress, this templating modification works only for pages. if you need to do this for posts, you need to install a specific extension "Custom Post Template" that will let you define specific post template, the same way you do for pages, by writing specific php template with instructions in the header comments.
Multi theming your site
Considering possibilities described above, multiple theming raises a lot of issues. The plugin JonRadio multiple theme comes with many configuration options to define specific theme for pages or sections. It has a big drawback though : it does not integrate theme options for the second theme, like widgets or menu, or other options.