Save multiple canvas as one image (make website like PicFrame)

I try to make website like PicFrame app so I found this demo :

http://ift.tt/1AXqm5P

I want to use multiple canvas in the demo and save it to one image. How to save all canvas to one images? Sorry for my english, Thanks!

My HTML code is look like this:

<div id="container">

    <input type="file" id="imageLoader_1" />
    <canvas id="imageCanvas_2" width="400" height="200"></canvas> 

    <input type="file" id="imageLoader_2" />
    <canvas id="imageCanvas_2" width="400" height="200"></canvas> 

    <a id="imageSaver" href="#">Save image</a>

</div>