Victor,Use the note mover tool. Write the measure in the upper staff, then use the note mover tool to drag the note to the lower staff.
You might want to become familiar with the 'special tools' tool. The reverse stem tool and the beam angle tool come in handy to give it a nice look.
Also, it might be necessary to write a partial measure in either staff- for instance, beat 1 + 2 are in the bass clef, part of beat 3 is in the bass clef, part in the treble clef. Write all of beat 3 in the bass clef, use the note mover tool to drag the notes in the treble clef. Then write beat 4 in the treble clef. (You will have to enter 3 rests in the treble clef, then write beat 4) Then use the special tools tool, reverse stem tool, and beam angle tool.
Hope this helps. If you get stuck, post another message with specifics.
Robert