Complex sentence: Rangers Coach Mike Keenan became the first person to head coach Game Sevens of the Stanley Cup Finals with two different teams, having been with the Philadelphia Flyers, a rival of the Rangers, in when they lost to the Edmonton Oilers.
Split into simpler sentences: Rangers Coach Mike Keenan became the first person to be a head coach in game sevens of the Stanley Cup Finals for two different teams. Keenan had coached the Philadelphia Flyers in when they lost to the Edmonton Oilers.