Can I put an ottava line indication on just a single voice on a staff?
Have you tried holding down alt/option when you add it?
Yes, see here.
(PS If I may take a small moment of pride, just now I went back to that page to add the keyword “ottava line” for the English manual, even though in Dorico we call them “octave lines”, only to see that I had already done so and therefore the search results for “ottava line” are already pretty relevant. Nice.)
@Lillie_Harris that’s because I mentioned this before about the manual! Brava!
Oh yes of course! Teamwork.