How to display in Vuforia for Unity backgroundPlane in ARCamera using another camera that only observes from the side?

Question

augmented reality

Inside the ARCamera hierarchy there is a backgroundPlane, which displays the video stream coming from the device's camera. The task is to display this backgroundPlane in a camera that exists separately (viewed from the side). But when you start the camera from the side does not see the backgroundPlane at all (as if it does not exist at all). How to solve this problem?
Thank you!