if you wanted to run the dual in dual out cat i posted before, you would have to remove the "collector" that makes the manifolds pipes merge into one, here are a couple pics of what you would have to remove (i just took these pics...its midnight and i go out to take pics for u lol)
As you can see from the pics, you wont need another o2 sensor, its on a smaller pipe connected to the two pipes coming from your manifolds.(you have enough pipe from there to connect a cat or pipes) the thing you have to remove kinda looks like a cat, but just splits your 2 pipes into 1, ill be removing mine when it get warmer out, and put true duals on, and ill take pics to show what/how i did it on here too