First assign your new piano part a different (polyphonic) score style, i.e. not the preset 'piano' but either 'piano 1/3' or the like, or make up a style for yourself. After you've done this, ALL notes will jump to the 1st staff. Then you have the possibility to use the 'double-arrow' tool (tiny arrows pointing up and down, in the toolbox) to draw the line you're referring at. Or (within the forementioned 1/3 polyphonic style) you might perform the tedious task of selecting by hand everything you want to see in the second staff, and in the collective parameterbox assign all those notes to channel 3.
or:
Create piano style with 2 voices each on treble and bass. Assign this to your sequence. Then select all and explode polyphony. You will get 4 staves and depending on your counterpoint, many of the note assignments will be correct. Then, and this is one great beauty of Logic, use the key command Midi Channel + or - to nudge notes up or down a staff. Also use the score style to assign stem direction ( I use Up for 1 and 3 and Down for 2 and 4, which gives a good start). Then unexplode polyphony to get 2 staves with correct voicing. This system is much better than it is in Sibelius, so if you're then going to transfer to Sibelius for print-out, do this in Logic first.