Ideally the o2 sensor would be just a bit before the collector flange (ie, in the header). Reason being that a leak at the collector flange can screw up your readings if the sensor is behind the flange. Of course you may not want to tap your brand new TTI's for an o2 bung and burn off your nice coating welding it in, so after the flange is where a lot of them end up. Just keep in mind if you're readings don't seem to be matching up with what your engine is telling you, check the flange gasket.
Having a bung on each pipe of a dual exhaust isn't a horrible idea. You don't have to use both of them at the same time, you can plug one and use the other. Just gives you more diagnostic ability, especially with a dual plane intake. The o2 sensor will tell you more than just how to tune your fuel, it can also help diagnose ignition problems (incomplete burn), etc. If you can isolate that to one bank or the other using the o2 sensor it can help speed up troubleshooting in some cases.
