Complex sentence: It was on that album that Kantner and Slick next worked with Pete Sears (who had first played on Papa John Creach's first solo album), who was co-producing a Kathi McDonald album in the same studio.
Split into simpler sentences: It was on that album that Kantner and Slick next worked with Pete Sears (who had first played on Papa John Creach's first solo album). Pete was co-producing a Kathi McDonald album in the same studio.