var context = canvas1.getContext('2d');
  context.drawImage(canvas2,0,0);

drawImage() for getcontext('webgl')?

ctx2D.drawImage(webGLCanvas.toDataURL("image/png"), 0, 0);

...getContext("webgl", {preserveDrawingBuffer: true});

ctx2D.drawImage(webGLCanvas, 0, 0);
// ctx2D.beginPath()...

<div style="position: relative;">
   <canvas id="layer1" width="100" height="100"
       style="position: absolute; left: 0; top: 0; z-index: 0;"></canvas>
   <canvas id="layer2" width="100" height="100"
       style="position: absolute; left: 0; top: 0; z-index: 1;"></canvas>
</div>