Hmmm, I'd put the weakest one closest to the power outlet just incase when it cuts off it does not briefly stop power to the desktop. Otherwise, I see no reason this can not be done. Seems more sensible to buy 1 bigger UPS, but I figure you've probably come across a deal or something.
IMHO though, these home UPSs are more intended to give you time to do a proper shutdown though. I doubt you're going to be playing Half Life or something for 20 minutes on them.
My '03 Sold.