Both are fantasy shows about romances between two people from different worlds who should have never met if not for a temporal shift in time and worlds. Both are fish out of water stories. Both sets of main characters have to figure out who is trying to kill them and how to survive all the attempts on their lives.
Both are fish out of water love stories. Both are about opposites attracting and having to overcome insurmountable odds in a dangerous world to be together. Both are fictionalizations of real worlds. Both are top 5 highest rated TVN shows & top 10 highest rated cable shows of all time at the time this recommendation was written.