Using
configSource
requires a physical file in either the same directory as the main config file, or a subdirectory. You can't refer to a file in a parent directory, and you can't use virtual directories to get around the limitation.
You could potentially create a
hard link[
^] to the common config file, which would make the same file appear in more than one directory.
mklink /h connections.config ..\connections.config
mklink | Microsoft Docs[
^]
Otherwise, you'll need to add the settings to the application-wide configuration. The easiest way to do that would be to use the "Configuration Editor" option in IIS Manager.