I don't know how GM that made that on the gmt800but I know from several German car brands, when they have to o2 Sensors, one before the cat and the other behind, the reason is, to measure the difference between this two sensors, to detect, if the cat element is working. So if there is a specific difference between these sensors, you receive no error.
If both o2 Sensors are working and the difference is equal, means the cat element don't change the exhaust gas at all, than you receive a explicit "cat error". That should as well be when the cat clogged and combustion is not that good because of the left exhaust Gas... That should also make and explizit "cat error" or no error.
But if one o2 sensor is electrically defekt, like the heating line has a shortcut, than you receive an explizit o2 sensor error.
I recall that I said, that I don't know how GM that made, but when they have two oxygen sensors, than they do that for the same reason and should handel it in the same way.