Another option (if you dont want to use the MIL) Go to your local muffler shop and have them weld in another 02 bung behind the second cat, should be about $15 tops. When you get home, lengthen your wires for the second 02 by about 4'...move the second 02 sensor behind the second cat and your cel should go away, this will be a sure way to ensure both 02 are giving different signals.
I have the Ractive header with no primary cat (obviously) and the second 02 behind the secondary cat...been that way for about 6 months and no cel.
Hope that helps.