I looked deep into this and did not find a great solution right in Visual Studio .NET. What I ended up using was my version control system for sharing folders.
I used Subversion Externals to share multiple working copies from the same source in Subversion.
http://svnbook.red-bean.com/en/1.1/ch07s04.html
I know there a similar concept of sharing in Source Safe and, possibly, in other version control systems.
source share