Satya's blog - Stupid UPS tricks
What would happen if you connected two UPSen in series? Like, plug one into the mains, plug the second into the first. Then when you lose mains, the first should keep the second going for a while, then the second goes to battery. I think it depends a lot on the VA ratings.
Now what about connecting UPA A to UPS B, and B to A? Would they run off each other? Infinite supply of energy? Blow up? I think, depending on the VA rating, they'd just discharge -- even if nothing else is connected. At what rate? Dunno.