Each study is given a Chart Region. So when you select your first CCI it gives it region 2. Then add another CCI study and the system automatically numbers it region 3 and shows it below 2 on the chart. You just have to go back into the settings and change the 3 to a 2 and it will overlay it.
The region setting is on the same screen that you set the CCI to 14 or 6, it's a drop down on the left hand side of that window.