Complex sentence: After touring for several years, playing in several festivals such as High Sierra Music Festival and CMJ Music Marathon, they decided to combine musical forces with their producers, Doniz and Samath, and they joined them on stage, playing guitar, and keyboards respectively.
Split into simpler sentences: After touring for several years, playing in several festivals such as High Sierra Music Festival and CMJ Music Marathon, they decided to combine musical forces with their producers, Doniz and Samath. The production duo joined them on stage, playing guitar, and keyboards respectively.