When you say “they will switch”, how are you doing this?
There are several ways …
You can use a composite envelope on the top track to reduce its opacity to nil when you want the lower track to show through.
You can Split the top event and delete the parts where you want the lower track to show through.
You can split both tracks and move the sections you want to retain up to a new track.
OR
You can automate the whole process by using Excalibur or Ultimatte S, making it extremely easy and fast.
Peter Wright
Perth, Western Oz
http://www.allroundvision.com.au