HTML5 - Sampling color from hidden image/video

I have a setup of two images, one in front of the other. When the user clicks on the front image I want to sample the same pixel in the background image.

In Actionscript 3 i used the bitmapData class to do this.

In the HTML5 document I can get the pixel color of the entire canvas but only the visible pixels. Is there a way to sample the image in the background that is not visible on the canvas? Maybe create a second context for the other image? Preferably without flipping images.