I will have to respectfully disagree with Steve on this one. If I understand the setting, we have a single service drop or lateral supplying an apartment building with two Services panels. First lets set the record straight, this is a single service with two sets of Service Entrances conductors.
The two Service panels each must have a GEC run from the interior of the panels to the Grounding Electrode System per NEC 250.64(D)(2). There are no provisions for going through another panel and using the "terminal block" inside the other Service panel.
I would say that a #6 CU should be run to the water pipe from the 100a. panel and not to the 200a. panel terminal block.