I need advice in placing grounding rods when upgrading electrical service
from 100 amp to 200 amp.

Code requires at least 2 5/8" x 8' rods placed no closer than 6'.

Soil conditions: sandy loam. Water table is about 15-18' below grade.

I am considering stacking rods so I will end up with 2 16' rods.

Question is how close can these be without compromising effectiveness of
grounding system?