Whether this will help you or not, I can't say - but my experience with two-sensor systems (on VR6 VWs) is that the rear sensor is solely responsible for monitoring the effectiveness of the catalytic converter. The value returned from that should consistently be about 0.2v, and any substantial variation would indicate the converter is not operating efficiently enough to control emissions, or some other condition is causing more unburnt fuel to be dumped into the exhaust than what the cat can handle - such as bad wires, or a faulty coil pack - or a faulty sensor. A fault in that should set a CEL (and possibly trigger limp-home mode - it does in the VW Motronic system). So it seems to me that the front sensor is the one you'd need to check since it's the one that would control mixture. 100k isn't all that old in my opinion, but it depends on whether any other contaminants - coolant, for example - got into the exhaust and shortened the life of the sensor. Sorry I can't be of more help.
--
Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina