You don't have to "hack".
In the "Add existing item" dialog, click the dropdown menu at the right of the "Add" button. You will see "Link" there.
The "Link" feature is what you need.
I use this often. In my project, I have 2 DLLs, but they are separated just becase whether it is shared by other proejcts or not.
So to keep logical separation in source code tree, I create classes to where it should belong to from architectually point of view.
And then if the class needs to be accessed from other DLLs, I use "link".