multiple o2 sensors. not so many EFIEs

alright. so i have a car that has 2 o2 sensors. the problem is i only have 1 efie. now before i go out and buy another 1, i would like 2 know if i can get away with installing just 1. and if i can, which oxygen sensor should i put it on? bank 1 or bank 2?
thanks

reading in some of the other posts about this you shold be able to get by with just the one efie. you will want to connect it to the o2 sensor before you cat. the other o2 sensor only tells the ecu if the cat is working right.