title: ExoPlayer - play two audio tracks simultaneously
body: <p>Using <code>ExoPlayer</code> i'm trying to play two audio tracks simultaneously.<br>
  My idea was to create two  <code>MediaSource</code>, and  to "combine" between them using <code>MergingMediaSource</code>.<br>
  This is what I have done:</p>
  
  <pre><code>    Uri uriAudio1 = Uri.parse(AUDIO_URL_1);
      Uri uriAudio2 = Uri.parse(AUDIO_URL_2);
  
      MediaSource audioSource1  = new ExtractorMediaSource(uriAudio1, dataSourceFactory, extractorsFactory, mainHandler, null);
      MediaSource audioSource2  = new ExtractorMediaSource(uriAudio2, dataSourceFactory, extractorsFactory, mainHandler, null);
  
  
      MergingMediaSource mergedAudioSource = new MergingMediaSource(audioSource1, audioSource2);
  
  
      mPlayer.prepare(mergedAudioSource);
  </code></pre>
  
  <p>How ever instead of hearing both audio tracks in parallel, i hear only the first <code>audioSource1</code>.</p>
  
  <p>Any ideas? Thanks!</p>