Please, follow the steps to create a popup inside popup:
1. Create any popup, except Iframe, Image and Video types (our example is with HTML popup)
2. Enter a Title for your popup (visible only to Admin)
3. Enter a "click text" for your popup 1 to trigger the popup 2
4. Highlight the "click text"
5. Click "Insert popup" above
4. Select the popup you need to be opened from the 1st one
5. Now it's inserted in your Popup 1
6. Save the changes and insert it on the page you wish!
Here you can see the result of the Popup 1 triggering the Popup 2.