Maybe they're just really good friends now? Exes can still get on if they're not forced to see each other's foibles on a daily basis 
I'm less inclined to cynicism with the royals (peculiar, I'll admit) so I'm inclined towards the view that the Foundation split is because of likely status changes when HM is no longer with us. Like has been said before, William will become PoW and have a very serious role and expectations. Maybe having completely separate charitable organisations will help keep the 'feel' different between the four, and financially allow differences that don't affect The Crown?
I enjoyed the throwback shot of William in aid of his birthday today. What a shmaltzy feel! Just to set the cat amongst the pigeons, the Sussexes wished a Happy Birthday to the Duke of Cambridge. Maybe they use informal for kids only? 