I found some similar tuplets in some Durand editions, it looks nice but never catched my eye. It seems like this is used only in cramped conditions. I have no clue on how to do this in MusicXML in a semantically correct way.
A slur is a curved line spanning two or more notes, not one.
Also, there is a certain score writer (won't name it) that wrongfully exports slurs starting and stopping on the same note, displaying your suggestion instead of deleting it would be worse.
I do agree that it would be nice to have this behavior, but I think MusicXML doesn't (yet?) support it.
If a note has 2 slurs with the same number:
start/stop would be an error (like I described above)
stop/start would mean a slur stopping, and another slur starting. They can have the same number since they appear next to each other. The number is used to make a clear distinction between slurs appearing above/below each other.
Michael, please correct me if I'm wrong
