If you use to see a piano score with the notes separated between a treble clef staff and a bass clef staff, the separation is automatic. You cannot move the notes between the staffs other than by transposing them.
To be able to see the right hand notes in a treble clef staff and the left hand notes in the bass clef staff, use separate tracks for the two hands. Then set the score view for the right hand to use only the treble clef and the track for the left hand to use the bass clef.
In GarageBand 10.2 open the track editor in score view.
Then click the clef in score view and click the clef you want to use for this track. You will only see one staff after that, either with the treble clef or the bass clef.
