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

First, in the Media or Photo part, click the Picture in Picture button (Add as Layer in Timeline Mode) on the lower-left corner of the video and you can add it as a layer to the canvas.

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

Unlike the picture PiP, you can add only one video layer to every storyboard/scene.