Complex sentence: Shortly before the Messina Conference (1--3 June 1955), Adenauer had given up his double post as Foreign Minister; and since Brentano had not yet been sworn in as Foreign Minister, Hallstein lead the German delegation.
Split into simpler sentences: Shortly before the Messina Conference (1--3 June 1955), Adenauer gave up his double post as Foreign Minister. Since Brentano had not yet been sworn in as Foreign Miister, Hallstein lead the German delegation.