Merge buffers

mergebuffers.js

// assuming `rec1` and `rec2` are both instances of Recorder... 

var buf1L, buf1R, buf2L, buf2R, worker, count = 0;

// start a new worker 
// we can't use Recorder directly, since it doesn't support what we're trying to do
worker = new Worker('recorderWorker.js');

// initialize the new worker
worker.postMessage({
  command: 'init',
  config: {sampleRate: 44100}
});

// callback for `exportWAV`
worker.onmessage = function( e ) {
  var blob = e.data;
  // this is would be your WAV blob
};

// get the left and right buffers from `rec1`
rec1.getBuffer(function( buffers ) {
  buf1L = buffers[0];
  buf1R = buffers[1];
  status();
});

// get the left and right buffers from `rec2`
rec2.getBuffer(function( buffers ) {
  buf2L = buffers[0];
  buf2R = buffers[1];
  status();
});

// just a simple/hacky way to tell if we've got buffers for 
// both Recorder instances (should probably be promises/deferreds)
function status() {
  if ( ++count === 2 ) {
    merge();
  }
}

// figure out the max length across each buffer, then loop
// through and sum all the values into two new left and right buffers
function merge() {
  var len, left, right, i = 0;
  len = Math.max.apply(Math, [buf1L, buf1R, buf2L, buf2R].map(function( buf ) {
    return buf.length;
  });
  left = new Float32Array(len);
  right = new Float32Array(len);
  for ( ; i < len; ++i ) {
    left[i] = ( buf1L[i] || 0 ) + ( buf2L[i] || 0 );
    right[i] = ( buf1R[i] || 0 ) + ( buf2R[i] || 0 );
  }
  record(left, right);
}

// send the new buffers to the worker, then ask it for a WAV
function record( left, right ) {
  worker.postMessage({
    command: 'record',
    buffer: [left, right]
  });
  // not sure if this is necessary or not, but it doesn't hurt
  setTimeout(function() {
    worker.postMessage({
      command: 'exportWAV',
      type: 'audio/wav'
    });
  }, 100);
}