Sorry - I figured it out!

I know this is an old post, but I'm where you were three years ago. How did you do this if I may ask?

Layers. The upper line in layer one, the lower in layer two. The stem directions will automatically adjust.

Thanks, but I really want TWO staves - the two parts are too interlocked and complex  - just up/down won't be readable. I've got two months - hope I find a solution.

Then, I don’t understand what you want. Can you explain more fully, or give us a pencil sketch?

Do you want two staves in some places, but one staff in others?

Just use two staves and combine them in a group.

Used two staves, no group. Combined in New Part and hiding empty staves did the trick.