Complex sentence: While growing up in Osaka, they trained at the Green Tsunuda gym, which also produced former WBC minimumweight title holder and former WBA interim minimumweight title holder, Katsunari Takayama, who also happens to be a critic of the Kameda brothers.
Split into simpler sentences: While growing up in Osaka, they trained at the Green Tsunuda gym. Which also produced former WBC minimumweight title holder and former WBA interim minimumweight title holder, Katsunari Takayama, who also happens to be a critic of the Kameda brothers.