assigning two instruments to one staff

Is it possible to assign two "instruments" to one staff. I would like to include both soprano and alto vocals on a single staff, but can't seem to find how to do this. I know that I can assign soprano to a staff, and create multiple voices. The problem is that the alto notes are outside the normal range of soprano notes and thus appear in red. I could live with this if I could figure out how to expand the range of "normal" soprano notes so that the range includes the alto notes.