[Ando140]

P0135 & P0155 are both heater circuit malfunctions, same thing happened to me. I had one go out, replaced it and the other one immediately went out.
I don't think the ECU compares the two sensors with regard to the heater circuit. I found in my service manuals that you can actually check the heater circuit with an ohmmeter. I can't remember what the resistance is supposed to be, but when I checked the old sensors I found the heater circuit was open and the new sensors were spot on the expected readings.
So, I think what happens is the heating element in the sensors actually breaks, thus opening the circuit. The fact that they tend to fail at the same time, I think, is a testament to the consistency of the quality (good or bad) of the sensors.