Adding second staff for only the last 12 bars of repeated section

I want to add a second staff for the last 12 bars of a piece. It is a repeated section, and I want a single instrument to play the top line the 1st time through, and the bottom line the 2nd time through. How do I create a second staff for only the last 12 bars?
Thank you!
***Note: I am using Version 1.3***