How to ground 400 amp service with 2 200 amp panels one on disconnect switch

At the main disconnect, is where the incoming Line neutral, the two neutrals going to the two panels, the two equipment grounding conductors, and the grounding electrode conductor all connect at one large multi-port lug.

The grounding electrode conductor will connect to a grounding electrode. If you have city water, or well water, with at least 10 feet of metal pipe in the ground, this will be your primary electrode. To this add at least one supplemental ground rod. With this method, you need to use #1/0 copper wire to the water and then to the ground rod.

If you don't have metallic water line to use as a grounding electrode, you then need to use at least one ground rod, most local codes require two rods, and with this method you can use a min of #6 copper wire to both rods.

Any connection between grounding electrodes must be continuous wire, no breaks or splices.

Be sure to use the insulated neutral bar in each panel for neutrals only, and add equipment ground bar bolted directly to the panel metal box using machine thread screws, for bare or green equipment grounding conductors only.

Do not install a bonding jumper at the neutral bars in the panels, keeping the neutral bar insulated and isolated from the metal box.

If you use the ground rod method, you need to "bond" any metal pipe system in the home with min #6 copper wire to the equipment ground bar.

I know, too many "if's". There are too many scenarios to address without knowing the specifics of your installation.