Nah breh you can't book a brand split with one world champion. It fukks up the flow and momentum of storylines and rivalries and it would be a clusterfukk of booking.
How would #1 contenders be decided? What if they want a world championship rivalry to go on for a couple months, what would the main eventers on the other show be doing? Twiddling their thumbs while waiting for the world champ to finish his beef on another show? How many main events can you book without a world champion before the show starts to feel less important? How would contenders even get at a world champion to start? It's difficult booking for no reason
And beyond that, you're essentially uniting the rosters when you have one world champion. Just think about it, everybody on both rosters are working towards the same thing, why even do a brand split at that point? It wouldn't work, it might if WWE changed its booking style and was more focused and detailed but they wouldn't and they aren't and they don't have to be when the simple solution is two world titles.