Suppose I am writing a part that can be played either on alto or tenor sax. I want to produce a complete, separate part for each instrument, with each part having the same notes in concert pitch but transposed appropriately for that instrument.
However, I would like only one of these instruments to appear in the score.
Is there a straightforward way of accomplishing this?
This is what the Clef and Transposition overrides are for in Setup mode.
You can create a new Layout, using the Saxophone player, but set a different transposition (and/or clef) as appropriate.
You could create an extra part layout for the sax player, with transposition overrides
[edit] @benwiggy was quicker 
Aha–that’s the terminology I was looking for. Just to be sure I have the terminology right: I have a single player, playing a single instrument, but I create two separate layouts for that player, and override clef and transposition in one or both of those layouts. Am I right?
Yes, that’s right.
Now I have a new problem.
I have a viola part that might be played by an ensemble with no violist but a cellist who can fill in instead.
My viola part is notated in viola clef (a.k.a. alto clef). I can create an alternative layout that I force to bass clef for the cellist.
However…because it’s a viola part, it’s really on the high side for a cellist to play. My cellist friend tells me that when I want a cello part to have more than four ledger lines above the staff, I should switch to tenor clef (which, by the way, is neither alto clef nor bass clef).
The problem is that the clef changes go in the part, not in the layout. So as far as I can see, there is no way to put clef changes in the cello part without screwing up the viola part.
Am I missing something? Do I really need to copy the entire part and then remember to keep the two versions in lock step?
Well, you can add Clefs to the staff, and they can be hidden Locally, per Layout.
So you would have to Hide the clefs in the Viola layout.
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Useful safety tip. Thanks.
I suspect that the reason cellists like to read tenor clef is that tenor clef puts middle C on the second line from the top of the staff and bass clef puts middle C on the first ledger line above the staff.
That means that notes in tenor clef are a perfect fifth higher than notes written in the same place on the staff in bass clef.
This difference of a fifth means that a cellist can use the same fingering in tenor clef as they would for notes that visually the same in bass clef–it’s just a matter of shifting over by one string.
That’s why the feature we were already talking about is called “Clef and transposition overrides”. You can tell the override layout to use another clef!
I get that. The problem I was having was that I wanted two Layouts from the same part, and I wanted one of them to have clef changes in the middle of it without those clef changes being reflected in the other.
Oh, I see. Yes, changing the clef somewhere right in the middle of the music is not “a thing” currently.
Sure–Dorico has no problem doing that. The trouble is that a clef change in the middle is part of the music, not part of the layout.
This is particularly a problem for cellists, because it is not uncommon for cello music to go well up into the violin range.
Take, for example, this passage from Dohnáni’s Serenade for string trioin which the viola plays double stops on open C and G strings while the cello takes the melody and soars over the viola. This passage would be torture to read (and write!) without the clef change.
But as Ben’s already said, clefs can be hidden locally from each Layout, so it’s perfectly possible to have a Viola part that looks like this:
and a Cello part, generated from the same Viola player but with Clef and Transposition Overrides, that looks like this:
It’s just a case of choosing which clefs to hide in which Layouts.
As to “torture” in your example, it really wouldn’t be - it only goes up to a G! A few lines later there’s an Ab, printed in bass clef in both the score and part. Cellists learn to put their fourth finger on a G three ledger lines up (in bass clef) as part of fourth position - the position that is learnt first following first position - so they really don’t have a problem reading three ledger lines in bass clef.
Sorry for being a bit unclear: I meant “not a thing” solely in the context of layouts in combination with clef and transpositoin overrides. 
@pianoleo Very nice workaround!
As to “torture” in your example, it really wouldn’t be - it only goes up to a G!
Fair enough–but I was just using that as a real-world example of a cello part that goes into the violin range.
The score I’m working with has stuff like this:
Hardly any cellist will have trouble reading this, even the high c, but this is typically better written in tenor clef.
OTOH: a clef change in an orchestral cello part may also subtly indicate a change of role: as long as the music stays in bass clef, the role is still that of a bass line, the bottom of the texture. A change to tenor clef could suggest a more solistic, non-bassy role in the expressive tenor register, while the actual bass line is taken care of by others.
Now I have a further problem.
I have a viola part, notated as usual in alto clef. I want to make another Layout from the same part to be notated in treble clef down an octave.
So…I go into Setup mode, make a new Layout, tell it to use the Player that I have in mind, and then–or so I thought–I just right-click this new Layout, select “Clef and transposition options,” and pick “Treble clef 8 below” in the “Transposed pitch” selector for that Layout.
I’m sure this has worked before. However, this time, the music remains in alto clef. This is true regardless of whether I’m in Setup, Write, Engrave, or Print modes. This is true regardless of whether I select “Concert Pitch” or “Transposed Pitch” in the lower left corner of the window.
I’m sure I’ve forgotten to set an option somewhere, but I’ve drawn a blank as to where. Can someone remind me?
Check if there’s an explicit alto clef at the beginning: if it turns orange, it is, and you can delete it, hopefully revealing the default alto clef.
Aha! An invisible clef. How do they get there?
XML-import?
Jesper
