Many cars have one O2 sensor before the catalytic converter and a second one after. The first measures the level of unburned exhaust gasses before the converter, and the 2nd one measures the gasses again after the converter, basically making sure the converter is doing it's job. If you have a harness plug for the other sensor, then go ahead and hook it up, if you don't, and you're not getting any codes or running issues, then I wouldn't worry about it, just leave it in there to plug the hole.
If you think 2 of them is bad, look at some of the newer cars, there are cars and trucks out there now with as many as SIX! lol
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
Turbocharged engine with two o2 sensors. Is this normal? 
