title: Overlay two audio buffers into one buffer source
body: <p>Trying to merge two buffers into one; 
  I have been able to create the two buffers from the audio files and load and play them. Now I need to merge the two buffers into one buffer. How can they get merged?</p>
  
  <pre class="lang-js prettyprint-override"><code>  context = new webkitAudioContext();
    bufferLoader = new BufferLoader(
      context,
      [
        'audio1.mp3',
        'audio2.mp3',
      ],
      finishedLoading
      );
  
    bufferLoader.load();
  
  function finishedLoading(bufferList) {
    // Create the two buffer sources and play them both together.
    var source1 = context.createBufferSource();
    var source2 = context.createBufferSource();
    source1.buffer = bufferList[0];
    source2.buffer = bufferList[1];
  
    source1.connect(context.destination);
    source2.connect(context.destination);
    source1.start(0);
    source2.start(0);  
  }
  </code></pre>
  
  <p>Now these sources are loaded separately and are played at the same time; but how do I merge these two sources into one buffer source? I do NOT want to append them, I want to overlay/merge them.</p>
  
  <p>explanations and/or snippets would be great.</p>