How would I need to go about to create that sub-bracket + descriptors you can see in the pic?
Buy serious notation software like Sibelius or Finale… 
Aren’t you the helpful one.
In Score Page View, open Score Settings>Layout. In the middle column, leftmost are the brackets.
This is one way to do it:
You would set a staff to polyphonic, with two voices active in each: upper and lower staves. You can set the clefs to whatever you want.
The notes for all four horns would go on the same track, with each horn on it’s own midi channel, which determines which voice the notes will be presented in.
After you have set that much up, back on the notation page double-click on the staff name to, and you will be able to add names for the upper and lower staves.
To get a similar look as your graphic, activate ‘show staff names to left of staff’ and ‘special braces’ in Score Settings>Project>Notation Style.
Thank you very much, Steve, I managed to replicate your procedure.
There’s also a piano part in the score which requires the usual “wavy” bracket, am I correct in assuming that I can’t have both, the “special” bracket and the “wavy” bracket in one score? Or is there a workaround?
edit: a bit clunky, but I managed to use the “User Symbols” to create the bracket.