Given a template where the HTML cannot be modified because of other requirements, how is it possible to display (rearrange) a div
above another div
Well, with a bit of absolute positioning and some dodgy margin setting, I can get close, but it's not perfect or pretty:
#wrapper { position: relative; margin-top: 4em; }
#firstDiv { position: absolute; top: 0; width: 100%; }
#secondDiv { position: absolute; bottom: 0; width: 100%; }
The "margin-top: 4em" is the particularly nasty bit: this margin needs to be adjusted according to the amount of content in the firstDiv. Depending on your exact requirements, this might be possible, but I'm hoping anyway that someone might be able to build on this for a solid solution.
Eric's comment about JavaScript should probably be pursued.