I support tranmere rovers and we have a reputation for being followed by liverpool and Everton fans. However, on an average 6k attendance I’d say 4k were 100% tranmere and 2k were a rotating cast of mostly liverpool or Everton fans, some who go to 1 or 2 games a season and some who go much more. But I think you’d be hard pressed to find an team who’s entire supporter base all supported another team.