Emit two sine waves, in stereo at different octaves.

def bisine_wave(frequency):
  """Emit two sine waves, in stereo at different octaves."""
  #
  # We can first our existing sine generator to generate two different
  # waves.
  f_hi = frequency
  f_lo = frequency / 2.0
  with tf.name_scope('hi'):
    sine_hi = sine_wave(f_hi)
  with tf.name_scope('lo'):
    sine_lo = sine_wave(f_lo)
  #
  # Now, we have two tensors of shape [1, _samples(), 1]. By concatenating
  # them along axis 2, we get a tensor of shape [1, _samples(), 2]---a
  # stereo waveform.
  return tf.concat([sine_lo, sine_hi], axis=2)