I did it with my N3ds xl usa, my cousin has a 2ds EUR, made a ctrtransfer on my n3ds and now it has an EUR firmware and we can play multiplayer via download play. So it's the only way. I tried once to replace the download play app by deleting it and installing the Eur version but it simply crashed everytime, since it has to match the firmware on the console. Dick move from nintendo to region lock local multiplayer in my opinion