The gang at XYZComputing is all about going beyond the bounds of safety & your warranty in this article; in which they explore the possibility of multiple batteries without (hopefully) getting electrocuted ;)

"Of course the most logical thing to do with two power supplies is to use them in tandem- plug your computer into one, and your monitor(s) the other, and so on. This method is safe, proven, and operates within the bounds of your warranty. But, being as it was safe, proven, and under warranty, my twisted mind was revolted at the idea. I thought to myself, "What if I was to daisy chain them?" By plugging UPS 1 into the wall, then UPS 2 into UPS 1, and then my computer into UPS 2, would I not be able to use the stored power from each?"