The video the dev made explains why he won't do this, as it would be incredibly complicated and a pain to make work, he plans on keeping each path separate. His video explaining all of this is in the Q&A Birthday post log
To answer your question, along with everyone else who ignores the video he made, he's thinking about it but there's problems with doing it. Not a "no", just a "not now", kind of thing. It might be possible to use Joiplay with a Godot plugin but I haven't tried yet