renaming app.config or referring it from subfolder

We have decided to upgrade our project from .NET 4.0 to 4.5.2 but want to keep support of .Net 4.0 as it is.

So we have created copy of each CSPROJ file in the same directory one for 4.0 and and other for 4.5.2, but they are referring to same app.config file. 

How can I have 2 app.config files for each project?