Extending iPad screen on iPhone?

I know that I can airplay my iPad or iPhone to a screen via Apple TV, and I know I can use my iPad/Phone as an extra display with apps like Duet and Astropad, but I'm wondering if there's anyway to my my iPhone (or another iPad) an extra screen for my iPad Pro? It would be great to toss screen off the side of my iPad into my phone and vice versa. Anything allow this?

Thanks!

C Powers