Adding a Piano (for rehersals only)

Hey all,

I have recently arranged a long choir piece and wanted to add a piano (for rehersals only), however I have noticed that once I copy a part eg: the soprano, and add it, and then try to copy/paste the alto into the same staff, the soprano dissapears.
Of course I could manually copy the entire alto, but as the piece is quite long, that would be very tedious. Just wanted to know, if anybody knows a way around this.

Thanks!