Complex sentence: The rivalry between the two clubs had grown more intense since the formation of the Premier League in 1992, since when all but one Premier League title had been won by Manchester United or Arsenal (Blackburn Rovers won the title in 1994-95).
Split into simpler sentences: The rivalry between the two clubs had grown more intense since the formation of the Premier League in 1992. The clubs had won all but one of the Premier League titles between them (Blackburn Rovers won the title in 1994-95).