Select voice 1 from the voice palette at the far right of the Score Editor control area and enter the notes for the top voice as you would normally. All the stems for these notes are up. Rests appear by default at the top of the staff, not in the middle, as they do when notes are entered with the default voice. Now select voice 2 from the voice palette and enter the notes for the bottom voice. For this voice, rests appear by default at the bottom of the staff.
You can change the level of the display of rests for a particular voice in a bar by changing the Rest Level option for that voice from the Display Bar dialog, available by selecting Bar from the Display menu.