You were almost there when you created your second project. Rather than adding the files to the second project, you need to link them.
When you add them, VS copies the source file to the current project's directory.
When you link a file, it leaves it in its current location and just adds a reference to the file to your project. This means that you are operating with a single source file instead of multiple copies.
To link a file, choose Add Existing Item...
menu item from the Project
menu, select the file(s) that you want to link, and then click the dropdown arrow next to the Add
button on the file dialog and select Add As Link
.
We have class files that are shared this way among a half-dozen projects, including Win Forms, Silverlight, ASP.Net, Services, and PocketPC.