Complex sentence: In 1963, the majorities of the two public factions of the Fourth International, the International Secretariat and the International Committee, reunited at the Seventh World Congress which elected a "United Secretariat of the Fourth International".
Split into simpler sentences: In 1963, the majorities of both public factions of the Fourth International (the International Secretariat and the International Committee) reunited at the Seventh World Congress. It elected a "United Secretariat of the Fourth International".