Split instrument staffs

I'm missing the function to split an instrument with two (or more) staffs into two (or more) instruments with one staff each. (I need this because I found out - to late - that parts cannot be created for a single instrument staff.)

Because the function is not (yet) available I tried to implement it for myself using a plugin.
But I failed.

No of the available samples is creating a new staff or instrument.
The documentation for plugin seems to be outdated and incomplete. (https://musescore.org/de/node/257436 does not give any information on the available API.)
All my attempts (reverse engineering based on github code) failed.
:(

Here my idea for the required actions (for two stuff instruments):
1. User must select at least one note of an instrument to be split.
2. The selected instrument will be duplicated (below the original one).
3. The second staff of the first instrument and the first staff of the second instrument will be deleted.

Who can help me to build such plugin?