Where is the optimal place to install a WBO2 sensor on a dual cat car? I've seen some people put it in the Y on single cat cars, but my Y is pretty far down the line.

 

Just about anywhere. Thou, the further back, generates a slight delay in response. WBs maintain a tight temp control of the sensor thur electronics so a min. EGT isn't needed for one to operate,

 

Mine is in the crossover pipe, on the left side of the engine, sort of in the region of the oil filter. Seemed to me to be a convenient location, reasonably well protected, easy access for the sensor, and easy to route wires to. I've had an NTK sensor in that location for about 18 months, and maybe 20K miles, with no problems.

I don't see any real need for the WB to see all 8 cylinders.