IMHO the most important one to put the extender on is the one between the engine and cat, as far as I know, its not a bad idea to put the one on the post cat sensor as well.
i agree if you put one upstream, that sensor will send one signal, and the sensor downstream(after the cat) will expect a certain change. unless you have identical setups on both sensors, then it will more than likely throw a code.
I remember reading somewhere that they shouldn't be used on the downstream sensors, but don't remember where I read it so I have nothing to back that up. I can't even remember what the reason for that was. I have heard of people using one per sensor and have also heard of people using two per sensor to move the sensor further out of the exhaust stream.