There are two ways for you to add a video PiP to the storyboard (the same as adding image pip).

First, in the Media or Video part, click the Picture in Picture button on the lower-left corner of the video and you can add it as a layer to the storyboard.

Second, drag and drop the video to the center of the storyboard directly.

Unlike the picture PiP, every storyboard can add only one video to create the PiP.