I6 EECs only have 1 O2 sensor input (V8s have 2) so running two would be pretty difficult to rig up. Only sensing from 3cyl isn't ideal but overall shouldn't cause any problems or difference in economy. Some extractor brands do this to try and keep the sensor at a hotter temp which increases accuracy.

when manafactors design and make extractors. they try and make them so the engine gains power and or fuel eco. they spend alot of money in designing them and testing them. like jmm spent a bit over 3 months testing and redisgning the lengths of the runners on his extractors. they put the o2 sensor there not for looks but becase that is where it works best with those extractors. dont move it leave it were it is. if realy worried go have a look at pacemakers, genie's, jmm and who ever else is making extractors these days u will c they all in different spots. been out of the exhaust buisness for year n half so memory is goin shottie on them..
but yer dont move it
we use to get ppl come in and want theres moved. so we use to put a buggered on in old hole and relocated good one.. knowing that they b back to get it changed back to original spot..

O2 sensor location is just a trade trade off. Some companies will try and get it as close as possible to factory location to get some heat into the sensor while others will place it all the way back and sense all 6 pipes (most use this method). Both have their advantaes and disadvantages and I don't see a problem moving it is you really want to, although I also see no benefit in doing so.