Complex sentence: His long collaboration with the Arden Shakespeare started in the 1950s, with the commission to edit "Hamlet", and then in 1958 he was named joint general editor of the series (along with Harold Brooks).
Split into simpler sentences: His long collaboration with the Arden Shakespeare started in the 1950s, with the commission to edit "Hamlet". In 1958 he was named joint general editor of the series (along with Harold Brooks).