Save multiple videos in custom designed layout in swift

I've developed a dual camera app in ios. It has this layout. While developing, I came across the challenge of saving the video in this specific layout. I couldn't think of any other way of recording the video so I came up with the solution of recording the entire screen and save it as a video. But recently I've seen apps that do not record the screen and yet they're able to save the video in a custom layout. One example of such an app is this. Now I'm unable to figure out how to make a video and save it in custom layout without screen recording(since its not perfect solution). So any help in this regard would be appreciated.