Hello! I would like to change the length of these stems in engrave mode, is that possible?
If you’re talking about the places where the noteheads ought to be, you need to find a way of selecting the invisible notehead and changing the “pitch”. Dorico isn’t aware that those noteheads are invisible - they’re still just regular notes with modified noteheads.
If you’re talking about the beams between the notes, just click and drag them.
If you’re talking about the gap between the staves, go into the Staff Spacing tool and move either or both of the staves.