Select the region you're working with.
Open the Staff Style window [Click-hold for the pop-up menu next to Style]
Set the upper staff (treble) to MIDI channel 1
Set the lower staff (bass0 to MIDI channel 2
(You now have a Polyphonic staff style which is essential for this to work.)
You'll probably find that all your notes are now in the treble clef area - that's because they are most likely all MIDI channel 1 by default.
Select the Voice Separation tool from the Tool menu (hit Esc)
Now just draw a separation line between the notes that you want to have in the upper and lower clefs. Doing so will change certain notes to MIDI channel 2 so that they display in the bass, while MIDI channel 1 notes remain in the treble.
You can of course also just change the MIDI channel of selected notes in the event list to achieve the same result.
Having 2 MIDI channels won't affect your sound output, unless of course you're sending to a multichannel instrument.
Hope this makes sense.....