Well that kind of defies common sense. If it's series rated with two, wouldn't more be better? Is there an electro-mechanical reason to this or is it just because it hasn't been tested this way for the listing?
It all has to do with dynamic impedance of the devices opening as well as the static impedance of the circuit. Because of this devices must be tested in series.
Square D does have three tier ratings for use with their multi-metering devices.