Split screen would be great, or even PIP with one stream the main focus and the ability to toggle between the active screen.
If it was easier to implement this as a separate app on the Apple TV, so it could hook into the existing server infrastructure, I could live with one active TVE full screen, with a secondary small TVE PIP screen the way the Apple TV already works.
The challenge might be figuring out how to toggle between the active audio and video stream (the one that has the focus)
I’m not sure Apple has put baked hooks into iPADOS so that developers can play around with this. hopefully one of the channels developers will read this and already knows the answer.
But even if it was a “static” PIP with two TVE streams going at once that would be an awesome feature.