Complex sentence: In 1937 he resigned from the "Times" to join the staff of "Time" magazine as radio critic, a position that he in turn resigned to join the staff of "PM", a left-of-center New York City daily.
Split into simpler sentences: In 1937 he resigned from the "Times" to join the staff of "Time" magazine as radio critic. He resigned to join the staff of "PM", a left-of-center New York City daily.