If you want the top line notes on the staff to have flags up and the bottom line to have flags down - it's easy.  Enter the bottom note first, with the flag down toggle on (shift and down arrow).  When you enter the top part, toggle the flag up before you enter the note (I have to press shift and up twice to make this work).  Presto! both divisi lines (i.e. duet) can be printed on a single staff chart.

Oh, yeah, to enter the top note, do the same as you would when entering a second note of a chord - use Control Enter.