395 wasn't anything like a constitutional division. It was intended simply as a division of authority between two emperors who were supposed to rule the empire together, each one in his part. That had been done before - you could argue that the Second Triumvirate makes it older than the Empire, in fact. What was different in 395 was that the two halves went their different ways rather than being reunited by war or heredity.
Had that happened earlier, I think we'd have mostly seen warfare between the two halves.