Using the Arrange - Keyboard Reduction Feature
1. Select all the music on the vocal staves.
(a) Press Ctrl+A to select all.
(b) Press Alt+U for plug-ins, then press Enter for the Access sub-menu, and
then press I for include/ exclude staves.
(c) In the list, select each staff which is not a vocal staff and  press the
change button. (You'll need to do this for each staff in turn, the dialog
does not support selecting more than one item in the list.)
(d) Press the close button.
2. Choose Edit > Filter > Notes and chords. This stops lyrics being copied
onto the piano staves in the later steps.
3. Press Ctrl+C to copy the selection to the clipboard.
4. Select everything on both empty piano staves.
5. Press Ctrl+Shift+V to open the arrange dialog.
6. From the list  choose "keyboard reduction: up to two voices per staff".
7. Press Enter.