Complex sentence: The presence of McCartney on the track marked the first time that he and Starr had worked together since the Beatles' break-up in 1970, reflecting an easing of the tensions that had existed between the two musicians for much of that period.
Split into simpler sentences: The presence of McCartney on the track marked the first time that he and Starr had worked together since the Beatles' break-up in 1970. Their collaboration reflected an easing of the tensions that had existed between the two musicians for much of that period.