This feature has been around as long as Windows 10. I've just never used it.
You can define a folder (or folders) in an app that can also be used by other apps from the same publisher and which also know about that folder.
Be careful of copying from the docs on this topic as they contain two errors in the sample XML.
The element name is PublisherCacheFolders (with an 's') and the Folder elements must be closed.
<Folder Name="Config" />
Just add the above to Package.AppxManifest for each project.
You can then access this folder from all projects with:
StorageFolder configFolder = ApplicationData.Current.GetPublisherCacheFolder("Config");