When Suzan-Lori Parks decided to write a play based on The Scarlet Letter, she began with the title: Fucking A. Unimpressed, she deleted everything she had and started from scratch, writing the play that would eventually become In the Blood. As Parks tells it, In the Blood had to come out before Fucking A would crystallize; she calls the plays “twins in the womb of my consciousness.” With both in revival at Signature Theatre, audiences have the chance to view Parks’ twins side by side. The plays are riffs on the theme of our duty to one another, colliding with and speaking to each other in a jazzy feedback loop. If In the Blood ends up the less viable of the pair, it still makes for a fascinating examination of the state the nation from a singular American voice.