Creating separate parts from combined staff

[arco]

I answered something like this here, it might be useful, using Edit > Filter options.

Estrarre parti in formato marcia

You can use the Edit > Filter to horizontally select your notes for each (depending on how they were input either as separate voices or as a stacked (chord)). If you were not consistent in how you input them (depending on different or same rhythms etc.) you might need to use both filters so you can cut/copy paste or [alt-M] as @MarcLarcher indicated. Edit menu > Filter, Different rhythms, different voices [Edit menu > Filter,] Same rhythm, stacked [same rhythm, stacked]