Like was said before... get a cheap O2 sensor to plug the second hole in your exhaust, pull your second O2 sensor out [still connected] and tie it up out of the way so all it'll ever read is fresh air. That should be plenty to trick your ECU into thinking all is well.

Or, you could put in a high flow cat and your ECU will read all is well as well.

Or, you could buy a little trick box that will do the same thing as the first method.

I believe that makes three methods to deal with your situation...