I install the O2 right after the collector on V8 cars. Read one bank at a time. There are some widebands that have two sensors. Unfortunately, you can't leave a sensor installed and not have it active. The sense element needs to be warmed otherwise deposit load up on it. A little swapping side to side has worked OK for me.
Putting the sensor the x pipe isn't the best place to install. You can't isolate a bank that way.
