Trouble with copy/paste

Hi,

I am trying to merge two staves into one by selecting / copying / pasting.

I was able to select all the notes in one stave, but nothing happened when I tried to copy and paste them (I had selected the first rest of the other stave before trying to paste).

If I cut instead of copying, the selected notes disappear, so I think the issue is at the paste stage.

All the notes in both staves are in voice 1. I am not sure if that could be the problem. I tried changing the notes of the stave I am copying to voice 2 first, but although all of them were selected, only some of them changed. If I then select a single bar, I can paste the notes into the other stave, but they replace what was previously there regardless of whether they are in the same voice.

I have had a look at the forum but I don't understand this behaviour and would be grateful for any tips.