I want to know the answer to the following question:
Who separated them?
All the background I'm provided with is that "Velletri was united with Ostia from 1150 until 1914, when Pope Pius X separated them again, but decreed that whatever cardinal bishop became Dean of the College of Cardinals would keep the suburbicarian see he already held, adding to it that of Ostia, with the result that there continued to be only six cardinal bishops.". Is that enough to answer the question?
Answer: yes