Complex sentence: For her second album "It's Not Me, It's You," Allen worked exclusively with Kurstin; he co-wrote every song and played all instruments on the record, which he also engineered and produced.
Split into simpler sentences: For her second album "It's Not Me, It's You," Allen worked exclusively with Kurstin. The two co ‑ wrote every song and Kurstin played all instruments on the record, which he also engineered and produced.