Who split roman empire into east and west?

Emperor Diocletian divided the Roman Empire into two parts, eastern and western, in A.D. 284. The eastern empire was controlled by Emperor Diocletian and the western empire by Emperor Maximian but Diocletian retained absolute power over both halves. Emperor Constantine built a new capital, Constantinople, where Byzantium used to be, in the east.

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During the years after Diocletian, the Roman Empire was reunited and redivided on various occasions. Constantine reunited it, and then divided it among his sons. The final division was in 395 AD, when Emperor Theodosius I died, leaving it to his sons Arcadius and Honorius.
It is unclear who was the first to divide the Roman empire in half i.e. the western part and the eastern part.( The Romans themselves never adhered to this thinking.) This "halfing" of the empire is just a device used by historians to clarify what part of the vast Roman empire they were discussing or comparing.