how to define “keys” ( the second input of export-musicxml ) to a grand stave ( piano stave) , the default input in LISP as ((g2)) only have a treble stave, I suppose there are more options in G F or G^ G F F multiple staves. thanks !
As i said before in this post to you: How to define key, approx in export-musicxml?
Unfortunately, you cannot define multiple staves for a voice. In OM a voice is one staff even if we can display it in multiple staves. It is only graphic. So you will need to do this “by hand” once you have exported your voices, or explicitly split your voice in n-voices and export it as a poly.
I know this because i wrote the code of export-musicXML.
Therefore, you should do this “by hand”, once exported, you open your preferred editor and add your staves manually. Then you get to do some cross staving.