Keep getting new PCM codes...help

My service engine light came on and after checking the code it said it was P0152, the #1 O2 sensor in Bank 2. So I replaced that with a Motorcraft sensor, reset the computer, and the Service light came on again. So I checked the code, and now I'm getting P0156, #2 O2 sensor in Bank 2. I have a feeling that maybe it's not the sensors, but could be an electrical problem somewhere. I'd like to think that it was an easy fix, and replacing the sensor would cure everything. But before I go out and buy another sensor, are there any other problems that could be causing this? I kind of have to question the fact that both sensors in Bank 2 would go out at the same time, but I guess you never know. The car's about to turn over 60k miles. Any help would be appreciated. Thanks.

OK, I'm wondering if I have an exhaust leak somewhere. Also, I had put a K & N Filter in, so I'm wondering if cleaning the MAF sensor would help. Would any of this cause those codes to occur? I apologize for my ignorance on this, but I'm not used to working on all of this electronic stuff.

Everything's stock on the car. It had a drop-in K&N but I took it back out because it seemed like the codes started after it was put in. The car drives fine except after it's been running a while and I pull up to a light it idle is a little off. I put in new (Motorcraft) plugs but it's still the same. So my next day off I'm going to check all the wiring to the O2's. I already pulled, if I remember right, the #2 and 8 fuses. Not sure if that's relevent to my problem but they were fine. Anywhere else I should be checking? Thanks for the help so far.