JPin,
Can you draw out exactly what you have? Or wish to discuss?
Simple diagram?
Above image is close enough. Any imbalance arriving on either neutral will not only flow along the neutral back to source, but will flow thorough the taps to the GEC, and over to the other panel's neutral and back to source. I'm NOT talking about current flowing down to the grounding electrodes and along ground back to transformer (the proverbial 25 ohm path). I'm talking about the path between the neutral bar in each panel, along the GEC taps and GEC.
Of particular interest is the sizing of the taps are a couple of notches smaller than the neutral feed, but will be carrying similar levels of current. In the example above, it is compounded even more by the different feeders to each panel. We have an 8 AWG tap on the first panel (sized based on the neutral feeder to that panel), but it may be carrying current imbalance off of the neutral from panel 2.
My initial query is whether anyone else sees this as an issue. I know it's code compliant. So I don't have an issue in terms of code. I presume when the code was written, this situation was assessed and deemed "OK". I'm just looking to see some of the logic behind the decision.
Edit: One other observation. I believe the phase delta add/subtract will be opposite along the GEC link. That is, same phase current (arriving at each neutral) will have a subtracting effect on the current observed along the GEC link. Opposite phase current will have an additive effect. It all comes out in the wash at the meter as the current heads for home. But one thing to note is you can have zero from meter to transformer and have substantial current along the GEC between the panel neutrals.