Even if you remove the catalyst, the front O2 sensor is required. The rear O2 sensor can be deleted. (The rear O2 sensor monitors the efficiency of the catalyst.) In other words, remove the rear physical sensor, tie up the wiring harness connector out of the way and have a tuner like Tom disable the rear O2 sensor functionality so you don't get a CEL.
Then plug the second hole or use it for your wideband sensor.