I have a Nvidia shield Pro (2019) and I'm trying to find a way to run two of the same apps (Plex) similar to what can be done on an Android phone os with work profiles and sandboxing like shelter. It seems like Kodi + Plex plugin may be the only option but would much rather not have to deal with Kodi if I don't have to.
You could try sideloading the apps that let phones do that like Island, Shelter, or Super Cloaner.
Then you could just make a second instance of Plex.
Edit: I see you already mentioned shelter. Try sideloadimg it. I doubt any have native leanback support, so you'd need to make a wrapper app from that special website or use a 3rd party launcher. And it still might not work.