Complex sentence: The 2016 season was FC Dallas'most successful season ever, achieving the double, by winning both the Supporters' Shield for posting the best regular season record and the U.S. Open Cup, the nation's domestic cup competition.
Split into simpler sentences: The 2016 season was FC Dallas'most successful season ever. They achieved the double by winning both the Supporters' Shield for posting the best regular season record and the U.S. Open Cup, the nation's domestic cup competition.