Are they heated OEM sensors, or just a single wire O2 sensor? With heated sensors you can put them in just about any section of pipe before the CAT and still get a reading. If they are unheated they need to be close enough to the engine for the exhaust to heat them up.
You also need to mount them vertical, in the range between about 10 and 2 o'clock so that condensation in the exhaust does not pool on the sensor. In other words the plug on the sensor needs to point relatively straight up, not down or to the side.