Complex sentence: They currently play in the Championship, the second tier of English football, after winning League One in the 2014--15 season, only the second team to win both the third-tier championship and Football League Trophy in the same season.
Split into simpler sentences: They currently play in the Championship, the second tier of English football, after winning League One during the 2014--15 season. In sealing the League One title, Bristol City became only the second team to win both the third-tier championship and Football League Trophy double during the same season.