When you asked this question, did you think about the world today? Isn't it interesting that we are still split calling ourselves Western and Eastern, having developed different cultures. In the case of Rome, the split was not due to different ideas or cultures: it was all Roman. But success had been too great. The Roman Empire, by the time of Theodosius, was immense coving most of Europe, North Africa, and parts of Asia.
It made communication and management too difficult. Before he died, in 395 AD Theodosius decided the answer was to split the empire West to East. His successor, Diocletian, carried this out. The Western empire was based in Rome. The Eastern empire, in Byzantium. Two separate emperors would rule their own part but work together against outside enemies. This worked well for the Eastern half of the Empire where spices and other exports brought wealth and stability. It continued for another thousand years after the Westen empire collapsed.