When you add an existing item from with the Visual Studio.NET IDE, click the small arrow on the ’Open’ button in the dialog that is presented. You will see a list of options. One of these is to ’Link file’. If you select this option then you will not get a local copy and any changes that you make to the linked file will be in the original file.
Another way to get the same effect is to share the files using Visual Source Safe. You can simply drag and drop the files between projects in VSS and they will also be in sync.