What are the requirements for splitting a sostenuto pedal line?
I tried in both Write and Engrave mode. Selected a notehead, right-click > pedal lines > split pedal line, nothing happened. From the manual it seems that only sustain pedal can be split but … why?!
Is there any way but shortening the line and inputting another one?
You could use a pedal line and replace one of the pedal marks with sost in Library→Music Symbols→Pedal Lines. Here, the P glyph.
Edit: See now that you’re talking about split and not retake, but you could do as below anyway.
Jesper
From this:
Split:
Thanks for this workaround. I was looking for a native solution or for an improvement to the existing behaviour because while this may work once, it would not work if both Sost and sustain need to coexist.
Yes, it will. You just replace one of the ped marks. Look at my first image. I replaced the p glyph, not the ped glyph.
Jesper
Or here with splits in both:
Sorry, I meant both pedal kinds at the same time (that is, with overlapping rhythmic durations).
Ah, I see.
Jesper
Yes, reason why I believe that two pedals of the same kind cannot be overlapped.
We’re going into philosophical ground here, though, since the original question was why the Sost. pedal cannot be broken into more pedals.
because technically a sostenuto pedal doesn’t work that way.
you can’t lift and retake and have the effect of that pedal. it’s not like a sustain pedal.
there needs to be a pause where you hold notes down before you can use the sost. pedal to give the mechanism time to engage.
a sustain pedal and a sostenuto pedal are not “the same kind of pedal”.
A sustain pedal lifts and drops all dampers on all strings.
A sost. pedal holds dampers that have already been lifted by striking those keys (silently or otherwise) then locks them in place. ONLY notes that were depressed at that time will hold/play. Which leaves the sustain pedal free to lift and drop and do whatever one needs with regular pedaling.
Sorry, Michel. When I said “the same kind of pedal” I was referring to the workaround proposed by @jesele , which was to use the standard pedal twice and then changing the appearance of one of them.
While I’m aware of the functioning of the 3rd pedal, I appreciate your most detailed description! 
as far as Dorico is concerned, isn’t there already a sost. pedal marking under keyboards in the playing techniques menu?
I didn’t create a new pedal marking for my example, I used the two already available in Dorico.
There is, indeed. But as @MicheleGalvagno is pointing out, it doesn’t support retake or split, unlike the normal pedal. I agree that it should support it.
I’ve liked and unliked this a couple of times - on reflection I think @Michel_Edward has a pretty good point even beyond the specifics of an acoustic piano. The difference in behavior of a Sost. Pedal applies for example to my Korg synth, where I’m sustaining previously held notes of the previous patch, while switching and actively playing new notes of a new patch (which the Korg allows) that I don’t want sustained. You can’t play that sort of thing like a retake though it won’t hurt me I guess if Steinberg changes Dorico’s behavior - I just wouldn’t write those using retakes.
The heart of it may be that we have a limited number of appendages and don’t like to fall down . What I mean is, I do actually have two sustain pedals (One per keyboard) on my live rig. I can write both simultaneously since I use different staves, but I’ve struggled with the compromises it seems like you have make on pedal and foot placement. It seems like one pedal does often end up behaving like a sost for reasons of my own agility - even if it technically isn’t. Does that make any sense? My thinking and foot position shifts if I’m treating it like a sustain pedal versus a sost/layer. So I think it’s a useful difference.
I was thinking about a case like this — not very common, admittedly, but it happens. Here, you’re supposed to do real retakes (with the left foot) on the middle pedal (faked here with a second piano and “sost” written as text on a sustain pedal):
That is a very clean representation of that situation
