Complex sentence: The rivalry between Benfica and F.C. Porto comes about as Lisbon and Porto are the two largest cities in Portugal respectively, and both clubs are two of the wealthiest, currently the most successful and influential football teams in Portugal.
Split into simpler sentences: The rivalry between Benfica and F.C. Porto comes about as Lisbon and Porto are the two largest Portuguese cities, respectively. These clubs are two of the wealthiest, currently the most successful and influential football teams in Portugal.