Attach your 1/0 from the water to the point where the 500's terminate in the CT cabinet and run one #6 from the rods to the same place. If the utility doesn't allow that, then I'd still run one 1/0 to both panels neutral buss and one #6 from the rods to both panels neutrals as well.
I have always considered it kind of funny that you base the size of the GEC from the largest set of service conductors, but there is no requirement to attach the GEC to the point where the largest set of service conductors terminate.
