250.64(C) requires a continuous (unspliced) GEC unless spliced by an irreversible compression connector.
250.64(D) pemits taps to a GEC where multiple service enclosures are installed.
If you enter the first enclosure and then run a jumper to the second one, it could be argued that the GEC to the second one is not continuous because it is "extended" (to the second panel) without the use of an irreversible comnpression connector as required by 250.64(C).
On the other hand, entering the first enclosure with #2, then jumping to the 2nd enclosure with #4, seems no different than tapping onto a #4 with a mechanical connector, which is acceptable for the tap (but not a splice)
It seems easier to just run an appropriately sized GEC (#2) beneath or above each enclosure and tap on a #4 to each ground buss.
[This message has been edited by Redsy (edited 04-12-2006).]
[This message has been edited by Redsy (edited 04-13-2006).]