Copying voice 1 part to voice 2 part?

There are two ways to do this. The examples show how to combine a soprano
staff and  an alto staff into a single combined staff. The methods are
different but the results are usually the same.

Method 1: the reduce plug-in.

1. Press Escape to clear the selection.
2. Choose Plug-ins > Composing tools > Reduce.
3. Find the radio button called use minimum number of voices.
4. If you want Sibelius to try and compress the notes into one voice (or as
few voices as possible) then leave this checked.
5. If you want the music from each staff to be created in a separate voice,
press UpArrow to select the separate all parts into separate voices radio
button.
6. Press the OK button
7. In the score, Select the soprano and alto staves from start to end.
8. Copy the music to the clipboard.
9. Select the staff in which both parts will be combined.
10. From the menus choose Plug-ins > composing tools > reduce.
The plug-in will take the music on the staves which you copied to the
clipboard and write it to the staff that you now have selected, arranging
the voices accordingly.

Method 2: copying and pasting between voices.

1. Select the soprano staff from start to end.
2. Copy these notes to the clipboard.
3. Go to the staff into which the parts will be combined and paste the
clipboard into it.
4. The pasted passage will still be selected. Press Alt+1 to move all the
selected items into voice 1.
5. Select the alto staff from start to end.
6. Press Alt+2 to move all of these items into voice 2.
7. Rests will have been created where the voice 1 notes used to be and the
passage selection will now include all the items in both voices.
8. Press Ctrl+Alt+Shift+2 to filter the selection so that it only contains
the voice 2 items.
9. . Press Ctrl+C to copy these items to the clipboard.
10. Navigate to the start of the combined staff and press Ctrl+V to paste
these items into it. Because the second group of items are in voice 2, they
will be pasted into that voice and the voice 1 items already there will not
be over-written.