wideband o2 sensor placement?
 

I have a 2.2T with stock heat exchangers, and a 2-in 1-out muffler. If I want to install a wideband AFR to help with tuning, where should I install the sensor, or do I require 2 sensors? The exhaust manifold doesn't converge before the muffler, and as I understand it, putting the sensor too far down the system will give me artificially lean readings.

I welded in a bung in each bank, one with the sensor and a screw plug in the other. The idea was to tune one side, move the sensor to the other, then tune that side. A little extra work but saves buying the extra $70 sensor. I don't foresee moving it much once it's been tuned and monitoring one bank is better than nothing.

Put it in the "secondary" single pipe of the header, immediately after the collection of three primary pipes coming together

Yep to be able to effectively tune each bank of three cylinders you technically ought to have an O2 bung on each header.