After some research I found that "Soares book on grounding", 7th edition, page 102 & 103 covers this GEC issue really well.
The method of sizing GEC is a matter of choice:
1. an individual GEC may be installed from each service disconnecting means enclosure to GE ( in our case one # 4 out of each 200A panel)
2. a single GEC is permitted to serve several enclosures. The main GEC is sized for the main service-entrance conductors ( 500 kcmil in our case). The NEC does not use the term main GEC to identify the GEC that connects directly to GE. It is only used here to distinguish from the grounding electrode tap conductor. Taps that are sized for the individual service-entrance conductors ( 3/0 in our case)are connected from each service disconnecting means enclosure to the the main GEC. ( one # 4 cu from each 200A panel will tap into 1/0 cu which is our main GEC)
TE I always use 2 rods not that they are required but just me being practical.
