both.scala

def both[F[_]: Concurrent, A, B](s1: Stream[F, A], s2: Stream[F, B]): Stream[F, (A, B)] = {
  def awaitCombine(s: Stream[F, Either[A, B]]): Pull[F, (A, B), Unit] =
    s.pull.uncons1 flatMap {
      case Some((Left(a), tail)) => 
        tail.pull.uncons1 flatMap {
          case Some((Right(b), tail)) => 
            Pull.output1((a, b)) >> awaitCombine(tail)

          case None => 
            Pull.done
        }

      case Some((Right(b), tail)) => 
        tail.pull.uncons1 flatMap {
          case Some((Left(a), tail)) => 
            Pull.output1((a, b)) >> awaitCombine(tail)

          case None => 
            Pull.done
        }

      case None => 
        Pull.done
    }

  awaitCombine(s1.either(s2))
}