I recently had a .NET solution where I wanted to use the same App.config in 2 different projects. I wanted to always ensure that changes made in the App.config of the original project would be reflected in other project(s) automatically. Listed below are the steps I used to facilitate that process.
1. Right-click your project in Solution Explorer
2. Select "Add" -> "Existing Item..."
3. Navigate to the file that you want to add to the solution
4. [Important] Instead of hitting Enter or clicking the Add button, you want to click the down-arrow icon at the right edge of the Add button, and select "Add As Link".