At the "edit dynamic video" page, click on the scissors below your video template

Next, highlight the text where you want a different background to appear and click the illuminated purple button at the bottom

Add the URL or media file you wish to have appear when the highlighted words are being spoken in your video, click continue

Click the purple save button at the bottom of the screen

You can regenerate previously generated videos (suggested) or save without regeneration

Now your videos will process and your multiple backgrounds will be active for all net new prospects added to this Dynamic Video