Well, yeah, take Hertfordshire and Essex, for instance....or Cheshire and Lancashire. But that's not the point, is it? Sussex was always known as 'Sussex', till about 40 years ago when it was divided between East and West Sussex. Why? What's the point? They seem pretty much the same to me. Both are known for their woodlands, hills, beaches and Areas of Outstanding Beauty. Both are rather posh and well-to-do, generally speaking, of course. Both are quiescently English, home to numerous quaint picturesque market towns with visions of cricket on the village green and Vicar's on bicycles. All the things that come yo mind when most people envision the prosperous South East of England and all the respectable middle-class propriety that accompanies such notions of the pristine Home Counties.
My own connections to 'Sussex' is through my maternal grandparents who were from Cuckfield, and the family itself has hailed from that county for as far back as we can trace. Nevertheless, I'm sorry to say my own knowledge of why the county was split into two separate counties is nil. Either way, it does not make a lick of sense to me why that split ever occurred in the first place.