Tom I was wondering the same thing as Pierre.
250.28 Main Bonding Jumper.
For a grounded system, an unspliced main bonding jumper shall be used to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor of the system within the enclosure for each service disconnect.
Both panels contain a service disconnect, both panels must be bonded.
Here is a graphic that shows the bonding, of course you may not have a trough but you can ignore that part of it.
You can choose to run one GEC based on the 400 amp service conductors than tap it at the panels based on the 200 amp service conductors feeding each panel. 250.64(D)
[This message has been edited by iwire (edited 01-18-2005).]