Double trill lines?

Dear list,

are there any tips on how to input a double trill line, one above and the other below the staff?



It’ll only work if the two pitches are in separate voices.
You’ll need to insert the second trill from the popover, with the caret invoked in the correct voice. Confirm the popover with Alt-Enter, then hit Space at least once (you won’t see the trill at all until you’ve hit Space once).