Complex sentence: John Cena and Seth Rollins would both go on to hold their championships until SummerSlam, where they competed in a "Winner Take All" match for both championships which was won by Rollins.
Split into simpler sentences: John Cena and Seth Rollins would both go on to hold their championships until Summerslam. At that event, they competed in a "Winner Take All" match for both championships which was won by Rollins.