You run one Grounding Electrode Conductor (GEC) from the bonded buss bar of either panel to the first ground rod, through the acorn clamp and on to the second ground rod. Unless the single rod can be shown to have an impedance to ground of 25 ohms or less the second rod is required. You then run a tap conductor from any convenient point of the GEC to the bonded buss bar of the other panel. The tap is connected to the GEC by a split bolt connector or a saddle clamp that does not break the continuity of the GEC. The tap conductor connection does not have to be irreversible. Are these two ground rods the only grounding electrode for your home?
I believe that the approach you outline is the correct one. You run one GEC to the largest disconnect that is sized for the equivalent area of the separate sets of service conductors. You then run taps from each of the other disconnect enclosures, that are sized for the service conductors that supply each disconnect, to the GEC. The only place I might part company is that the GEC itself need not be larger than the size required for the conductors between the service drop or lateral and the meters line terminals. Keep in mind that the conductors between the meter and the individual disconnect enclosures are based on the size of the disconnects that they supply and the size of the conductors on the line side of the meter are based on the computed load. Since the code specifically permits the separate sets of overcurrent protection to add up to more than the ampacity of the conductors on the line side of the meter the equivalent area of the load side conductors may be greater than the actual area of the conductors that carry the entire load to the meter. That is why I said that the conductors on the meters line side are the service entry conductors and the conductors on the meters load side are in reality service entry tap conductors.
Your point is well taken. When the lateral extends to the line terminals of the meter enclosure or CT cabinet there are no common service entry conductors and you would size the GEC by the equivalent area of the multiple conductors on the load side of the meter. In those installations were there is a demarcation point on the line side of the meter then the size of the conductors between that demarcation point and the meters line terminals would govern the size of the GEC. The demarcation points that are written into many state public utility tariffs are the splices between the drip loops of the service entry conductors and the service drop, or the splices that are made in the property line hand hole or the underground lateral terminal box between the service lateral conductors and the service entry conductors. Terminal boxes are used at the point were individual type USE service lateral conductors enter a building and type SE cable or raceway is used to carry the current to the line terminals of a meter enclosure or CT cabinet.
