How can I get in Dorico a second Part from the same player e.g. Trombone in C Bass Clef and Trombone in Bb with Treble clef ?
´
Welcome to the forum, HZs. Dorico doesn’t have a good way of producing multiple layouts for the same instrument with different transpositions as yet. For now your best bet is probably to add another trombone instrument to your project, copy and paste the music from the existing trombone part to the new one, and then exclude the new trombone from the full score layout.
Dear Daniel,
thank you for your work around. With this workaround there’s a lot of effort when you change something in one part without linked to the others.
Here’s a example, where you have to change up to four parts, when you change one part.
May you can do this feature as a request for the next version. Linked Parts are aready possible in Dorico 3 Pro, you only have to program that the clef and Transposition is only valid for the defined part…
This is one of the most used functions for orchestras. That’s why years ago a lot of musicians changed from Finale to Sibelius…
Best Regards
Hubert
Hubert, this is already firmly on the backlog but don’t expect it for “the next version”. The next version is imminent, and this feature hasn’t been promised for it.
If there was a feature voting system for Dorico, I’d give my “+1” here 
Indeed this is one of the main reasons that keeps me from Dorico yet when doing leadsheets and arrangement for my jazz combo. If my leadsheet is already finished and there’s a player/layout in C it should be possible to quickly create and print another version in Bb or Eb for the sax/trumpet players. The current workflow (duplicate the player, copy/paste the notes, copy/paste the chords, delete system text which has been pasted twice and copy the layout to the new layout) is very tedious. Or in a more constructive way: It would be awesome to have this feature in Dorico!! 
It’s slightly less tedious to:
Duplicate the player
Select all on the original player
Edit > Filter > Select/Deselect
Edit > Filter > System Text
Copy
Paste to the new player
Ensure that the new player has appropriate settings in Setup mode.
That’s true… thanks!