So you want Caliphate and Byzantine Empire to become coexist under the same monarch ? Well then what do you have to do first, is to find a right PoD that the Caliphate will conquer Constantinople, without explicitly abolish Byzantine Empire! And this task will be far than easy, let alone simple. I seriously doubt that there was ANY chance to motivate any Caliph to officially take the throne of Byzantine. Because why should they ? Would such act even gonna make sense anyway ? But if some folks in Byzantine Empire, or no, even SOMETHING that able to manage to provide the reason for the Caliphate to do that, to make the Throne of Constantinople a very valuable price to fight for, then you should start anything from here ! Honestly, this is exactly on the borderline of ASB (if not more than a nanometer already entering ASB territory)! Nevertheless the consequences of such PoD will definitely worth a time line.
If you even dare, to manage to actually figure out a way to do this without any interference of an even single ASB element, then I'll make a fatwa declaring you as "inevitably going to Hell" if you're not gonna make any TL based on this idea!!![]()
I accept the challenge! Now I've got to go find a good POD....