If you are bound and determined to have separate projects, then you can use Portable Areas, which are part of the MVC Contrib project.
http://mvccontrib.codeplex.com/
However, you do not need to have separate projects just to have different UI's. By default, Areas have their own Views and Shared Views areas, and use separate master pages. So you can use normal areas and still maintain two separate looks and feels, and different code, and anything else.
The only reason to use separate projects are:
- You have different people working on each and want to keep them physically separate for ease of development.
- You want make a portable area be a "plug-in" sort of thing, which it sounds to me like that isn't the case.
- You want to deploy the areas separately from the main app.