the easiest way is to use position:absolute on both elements. You can absolutely position relative to the page, or you can absolutely position relative to a container div by setting the container div to position:relative
<div id="container" style="position:relative;">
<div id="div1" style="position:absolute; top:0; left:0;"></div>
<div id="div2" style="position:absolute; top:0; left:0;"></div>
</div>