I’m writing a work for a rather big fanfare orchestra of some 60 players (brass, saxophones, percussion). De brass players don’t like reading sharps. And this gives me a problem for the horns in F.
There are a few passages in G# minor, which means D# minor for the horns, with six sharps, which is not preferable. But I see a solution: the enharmonic E♭ minor key has six flats, the same amount of accidentals, but now flats instead of sharps. Problem solved! Well, that’s what I thought. And this is what I did.
I first chose the layout with only the horns and then changed the key signature with Shift+Key, writing eb for E♭ minor. But I got just five flats instead of six as key signature. So I recon that it still refers to the overall key signature, that is, the one for the intruments in C.
The overall key signature here is G# minor, which means that, when I want the horns to get six flats, I must change the overall key signature to A♭ minor. But, this makes the instruments in C get seven flats as key signature, instead of five sharps…
WHAT I NEED HERE is the possibility to place the horns in E♭ minor and keep the overall key signature in G# minor. Then the E♭ instruments get 4 flats, the B♭ instruments get 5 flats, the F instruments get 6 flats and the C instruments get five sharps.
BUT HOW DO I DO THAT??
Maybe experiment with local key signatures on the brass instruments if it’s not a problem if those are visible to the conductor too?
Yes - here are the specific instructions for inputting separate key signatures, from this page:
Input the key signature and close the popover in one of the following ways:
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To input a key signature on all staves, press Return.
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To input a key signature only on the selected staves or staves across which the caret extends, press Alt/Opt-Return, or activate Local/Global
then press Return.
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Thanks, Ton. Well, if I change the key signature for the horns towards E♭ minor, I would indeed need the conductor to get the same key signature for the horn section, because otherwise they could get communication problems.
So I’ll try the link and also the one that Stephen gave me (thanks also to you, Stephen). I hope it’ll work. (Otherwise I just maybe come back to you… 
)
Well, it worked. But I still experienced something odd. I activated the first notes in all horns of the fragment that modulated towards G# minor, where the horns got the 6 sharps. I typed Shift+Key, wrote eb (for E Flat minor) and tapped Ctrl+Alt+Enter (in Windows). Indeed, only the horn section changed, but with 5 instead of 6 flats, which means that I stil have to write what would be the overall key signature, even while it’s only for the horns…
Of course in most cases (French) Horns do not show the key signature on the staff but use accidentals instead. That should solve any problems about their key signature.
I think this convention largely died out many decades ago. It is still used in modern editions of older scores.
Not sure. In a score I once sent Philip Rothman in NYC for printing and binding, he suggested I keep the score in atonal so that all the parts used accidentals only. I had asked him to make any suggestions his experience suggested to him.
The score was for a musical show and was largely tonal.
Still, thank you for your observation.
There are probably different conventions in different ensembles / genres, but for a fanfare orchestra or wind band I’d definitely expect key signatures in the brass (including horns) if the woodwinds have them.
@DirkMM
It works using Clef and Transposition Overrides… (which you can customise for the desired layouts, just using the enharmonic equivalent note).
You may want to use the Player name as Staff Label (in Layout Options>Staff Labels) for the Horn in F (otherwise it will display Horn in E#); you can rename the Player as desired.
Dorico file example:
enharmonic key - transposition override.dorico (1.4 MB)
Definitely not the convention for Dutch/Belgian fanfare bands that OP is writing for in this case (they get the same treatment as the rest of the band).
Came here to point out the clef/transposition overrides for both score and parts but of course @Christian_R beat me to it. 
Indeed. And BTW, IMO, when one writes tonal music, I think it’s will always be preferrable to note the proper key signature. I don’t believe anybody can say that it’s no longer appropriate to do so.
AND Dorico provides this (of course), a lot of composers would not use a program that doesn’t.
I STILL HAVE A QUESTION:
I succeeded, with the help in this conversation, in changing the key signature for one instrument or a section. But going from sharps to flats, the notes were not adjusted along with it. They retained all sharps as accidentals for the notes. Which still took a lot of work to change them into their enharmics within there ‘new key’. Is there a possibility to have the notes adjusted in a single action, together with the change of key?
There are Respell functions in the Write menu. If you select the notes you want to fix, and run Respell → Reset Note Spellings in All Layouts, that should do the trick?
Well, yes, Rikard, that’s what I did. But it’s still quite some work and also I need to check because often the logic of the key signature isn’t followed, resulting in double sharps or flats…