Bad snippet! Don't do it.

BadBidiFlow.scala

object BadBidiFlow extends App {

  implicit val system = ActorSystem()
  implicit val ec = system.dispatcher
  implicit val materializer = ActorMaterializer()

  val source = Source.fromIterator(() => Seq("1", "2a", "3").toIterator)
  val sink = Sink.foreach(println)

  val endFlow = Flow.fromFunction[String, (String, Try[Int])]{ a => (a, Try(a.toInt)) }

  val graph = BidiFlow.fromGraph(new BidiRetryGraph).join(endFlow)

  val runnableGraph = source.via(graph).to(sink)

  runnableGraph.run()

  class BidiRetryGraph extends GraphStage[BidiShape[String, String, (String, Try[Int]), Try[Int]]] {

    val in0 = Inlet[String]("in0")
    val out0 = Outlet[String]("out0")
    val in1 = Inlet[(String, Try[Int])]("in1")
    val out1 = Outlet[Try[Int]]("out1")

    override def shape: BidiShape[String, String, (String, Try[Int]), Try[Int]] = BidiShape.of(in0, out0, in1, out1)

    override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new GraphStageLogic(shape) {
      setHandler(in0, new InHandler(){
        override def onPush(): Unit = {
          push(out0, grab(in0))
        }
      })

      setHandler(out0, new OutHandler(){
        override def onPull(): Unit = {
          pull(in0)
        }
      })

      setHandler(in1, new InHandler(){
        override def onPush(): Unit = {
          val res = grab(in1)
          res._2 match {
            case Success(_) =>
              push(out1, res._2)
            case Failure(e) =>
              val n = res._1.filter(c => c.isDigit).mkString
              push(out0, n) // requirement failed: Cannot push port (out0) twice
          }
        }
      })

      setHandler(out1, new OutHandler(){
        override def onPull(): Unit = {
          pull(in1)
        }
      })
    }

  }
}

           +----------+           +----------+
           |          |           |          |
----------->   |----->------------>          |
           |   |      |           |          |
           |   |      |           |          |
           |   |      |           |          |
<-----------   |-------<-----------          |
           |          |           |          |
           +----------+           +----------+