The easiest way would be to loop a GEC, based on the conductors feeding the panels (250.66) down from one to the water pipe, through the clamp and up to the other.
You can size to each panel. The other option is to run a GEC sized to the SE conductors (250.66) up towards the panels and splice the appropriate GEC from the panel to that.

From the handbook
[Linked Image from gfretwell.com]

Greg Fretwell