Normally a tuning shop will use their own wideband O2 sensor to tune your car. They turn off the factor O2 sensors to disable "learning" while tuning the car, and install their wideband sensor into one of the factory O2 sensor bungs, or sometimes with a tailpipe connector (though it's more accurate placing it where the factory sensor goes).

Depending on the modifications made to the car, they might switch their wideband from one side to another so they can get data logs from both banks.

If you have a boosted car, they may request or require you run the car with a wideband O2 sensor after it's tuned so you can monitor air/fuel ratios. When I tuned for people back in the day, I never required them to have a wideband O2 sensor in the car. I used my own. Only if I was doing a remote tune where I didn't have hands-on access to the car would I require them to have a wideband O2 sensor but usually only for boosted cars or setups where I knew the narrowband factory sensors would give sufficient information for their build. I know many tuners in the industry and don't know of any who require a customer to have a wideband O2 sensor installed as a condition of tuning it.

Maybe there's a good reason they require it, I don't know. But it sounds odd. Like they don't do many cars or they otherwise aren't doing enough business to afford something as basic an having a shop wideband O2 sensor on hand. Its something any tuning shop ought to have on hand, period. It's especially odd because not all wideband setups connect the same to data logging equipment, and they'd have to wire it in once you got there. Or they are tuning strictly by what your wideband gauge shows them, which is a bad sign because it will not log the exact readings along with RPM, throttle position, etc.

If you do install a wideband, usually one is sufficient for most cases. If you're running a radical setup, one on each bank might be worthwhile.

 

You put it downstream near the regular O2 sensor as that distance has the optimal amount of heat for good operation. It should go at the top of the anywhere between the 10 pm and 2 pm position, which keeps condensation from building up in them as the exhaust cools off. Never closer than 2 feet from the engine.

You don't use a single O2 to read two banks with a y, x or h pipe. You read one bank only. If you go that far downstream from the engine the sensor can foul because its too cool. Its okay to only read from one bank.