When copying from a 2-voice bar with coincident unisons, the note in the uppervoice is not copied.
I hope the screenshot shows the problem, green bars are copied (hitting R or Ctrl-C/Ctr-V, doesn’t matter).
Ludwig
Ludwig, you’re quite right. This is an unfortunate consequence of two aspects of Dorico’s design: firstly, you can only copy what you can select; and secondly, when Dorico suppresses a notehead because it is coincident with a note in another voice, it is not drawn, and you cannot select that which is not drawn. So unfortunately you can’t select the stem-up notes in these situations, which prevents them from being copied.
We think something should be done about this, but we’re not sure at the moment what approach should be taken to sort it out. It’s on our backlog.
This seems somewhat related to the fact that when you hide the 3 of triplets, you can’t select the either. So when you copy and paste a passage, you will end up with regular notes instead of triplets.
Provided you have the signposts visible, you can select them, and that will allow the tuplets to be copied as well as the notes they contain.
So hidden unisons need a signpost too, to be able to copy them 
I would be very unhappy if Dorico would introduce signposts for that. Conceptually, both notes are just there. Neither of both is meant to be hidden. It’s just that Dorico’s drawing engine decides that two overlapping notes may be drawn as one glyph, but, TBH, I think that shouldn’t concern the user at all.
Daniel,
thanks for your answer (on Sunday!). I am looking forward to your solution …
BTW, the work you have done so far with Dorico for guitar players is amazing.
Ludwig