JoeN:
Forgive my total lack of basic electrical knowledge, but doesn't wiring these in series actually make the voltage through the whole circuit including the capacitors themselves 5.4V and exceed the specification of the two capacitors?
If you supply a regulated voltage of 5v to TWO caps in series, each cap should display 2.5v, the voltage in = 5v, you can't get any higher than that, if the Cap's were fully charged that's different then you'd have to discharge the caps to 5v, then i'd use it...
But I'll take a couple of precautions when using with atmega, and I think a Diode (.7v drop should do it)