Is there a way to lock an app into spanning?

I read a lot on the Kindle App and love how the Duo works with the app spanned between both displays. I thought I saw somewhere that you could lock an app into that posture but can't for the life of me remember where I saw it or figure out which setting does that.

That said, I'm on the Duo 1 and loving this device. Slight bug here and there with the UI but nothing more than what I had with my Samsung S10. What an upgrade.

Thanks for any help you can offer