Isn't 250.64(D) a permissive statement and not a requirement? Without 250.64(D), I'd think that this would not be allowed because this creates a parallel neutral and causes "objectionable currents" to flow on the common portion of the GEC (violates 250.6(A)). Yes you can run GEC taps to each panel, but I don't see where it is required.
Can't the service neutral that interconnects the two panels be considered a GEC (based on 250.64(C) exception and if it is sized large enough) and suffice for bonding (based on 250.142)?
If it was me, I'd run a GEC to your rods and a GEC to the water pipe each from the 200A panel. Do watch the size requirements as specified in 250.66 and be careful if you used parallel service entrance conductors (and you have to pick the largest ones, not just based on the size feeding one panel). For my 400A service, I used 400 KCMil copper SEC's and this required a 1/0 copper to the water pipe. If parallel 2/0 copper wires were used in my mast instead, I could have used #2 copper for the GEC.
