With a POD after the Shia-Sunni split, have both the sect adopt one of the holy cities as their holy city. Also both should maintain that the other city is the lesser holy city.
I don't see this happening. Both cities are for Muslims important because another is Mohammed's birth and home city and another is the prophet's death and burial city. And Mecca is bit holier. So there hardly is way that they would begin arguing about both is only holy city.
Both are holy cities, but the sects maintain that one city is more important than the other.
I don't really think this is possible given that there's only one set of the holiest sites in Islam, and they're all in Mecca. I can't think of any reason anyone would view Medina as holier.