Complex sentence: It brought together two organizations: the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) and the Council for Secular Humanism (CSH), both of which had previously operated in tandem but were now formally affiliated under one umbrella.
Split into simpler sentences: It brought together two organizations: the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) and the Council for Secular Humanism (CSH). CSICOP and CSH had previously operated in tandem but were now formally affiliated under one umbrella.