This doesn't really appear to be related at all to Joplin itself?
From what I understand, "Dual app" is some kind of specific name for an app cloning application? Where it puts its data would be entirely down to that application itself, I'm guessing it just emulates the proper file locations to somewhere else.
Also it doesn't seem like you are syncing using anything to do with the application itself? Rather just copying the full application directory/database beween devices which isn't a supportable method and could very easily lead to some weird conflicts and data loss depending on what systems are accessed and when.
Yes, true. Folder location might be dependant on device only, but that is something I don't know, as eventually location might be dependant on device & app altogether.
Standard location for Dual app doesn't have Joplin folder
Good to know that SYncthing is not supported although it is working fine for over 2y for me.