OK, I've found out that staves can be moved in the Score Setup. I didn't find out before because the manual says:
"Drag to a new location (between existing staves) until you see a thin, light orange bar between the staves where you want the instruments to appear ."
But there is NO light orange bar when dragging a staff - just a very light-gray bar which appears over a few pixels of dragging space. Maybe the orange color has been lost somewhere?