I currently have my o2 sensor placed into only one side (passenger), instead of it being after they both come together. Would this cause it to run lean?

 

Mine is that way too and its not usual for it to as long as the front and back on the same line (that may not even make a difference)

 

I have Pacesetter LT's... the upstream O2 is in the passenger side collector... I have the downstream O2 after the Y-pipe just before the muffler.

I added a sensor bung to the drivers side collector to install a Wideband O2.
The AFR readings are just fine.

 

Makes sense since I still have a backfire.