Use CSS position: absolute; followed by top: 0px; left 0px; in the style attribute of each DIV. Replace the pixel values with whatever you want.
You can use z-index: x; to set the vertical “order” (which one is “on top”). Replace x with a number, higher numbers are on top of lower numbers.
Here is how your new code would look:
<div>
<div id="backdrop" style="z-index: 1; position: absolute; top: 0; left: 0;"><img alt="" src="https://stackoverflow.com/backdrop.png" /></div>
<div id="curtain" style="z-index: 2; position: absolute; top: 0; left: 0; background-image:url(/curtain.png);background-position:100px 200px; height:250px; width:500px;"> </div>
</div>