Changing the exhaust to true duals usually doesnt alter the A/F ratio. What actually causes the P0420 in most true dual setups and header installs is actually the location of the sensors and the new exhaust flow path.
From the factory, the PCM is calibrated to the signals the o2s should read with them in the factory location, with all the exhaust flowing through the factory tube down the factory path. When you start to change that path, tube size, distance between sensors, distance between cats and sensors, etc the data that the o2s will give off starts to vary. The truck isnt actually running any differently, but the DTC detection logic interprets the difference in data as a catalyst efficiency problem and throws the 420.
Now when you throw codes for the front o2 sensors, you will have a change in A/F ratio. but the catalyst effic. codes in your case are usually because of the new location of sensors, or the new flow path.