normally i would replace both at the same time. it is a little more money to do it, but it is well worth it. I have seen the second bank throw codes right after replacing the one causing the problems.
just my thoughts. i know that others have only replaced one and not had a prob. how many miles you have on the engine will also be a factor in this decision. also pull a plug and check for fouling. It may be something a little more than an o2 sensor.