Complex sentence: When violist Zheng Wang left the group they had trouble finding a replacement violist, and upon accepting Yi-Wen Jiang, who the Li brothers had already known, as the second violinist, Honggang learned to play the viola in order to complete the quartet.
Split into simpler sentences: When violist Zheng Wang left the group, they had trouble finding a replacement violist. Upon accepting Yi-Wen Jiang, who the Li brothers had already known, as the second violinist, Honggang learned to play the viola in order to complete the quartet.