Installing a second electric meter. Wiring and grounding questions.

I have a rental house in CA with a separate in-law house. I have a permit and I am going to install a separate meter/combo next to the existing house meter/panel (per the utility company's engineering department) to power the in-law house. The in-law house currently has a subpanel on the outside which is fed with 220v from the main house meter/panel with two 20 amp breakers that power two 20 amp breakers in the subpanel for lights and outlets in the in-law house (It doesn't take much power for the in-law; it is only 400 sq ft, with about 6 outlets and 3 light fixtures and a refrigerator. Stove and water heater are gas). Two 20 amps, as it is, should be sufficient. Do I keep it wired the way it is with two 20s from the new meter panel feeding two 20s in the existing subpanel? As far as grounding, can I use the same grounding rod for the new 100 amp panel that the existing house panel uses? The subpanel has it's own grounding rod located outside the in-law house that is currently connected. I thought a subpanel does not usually have it's own grounding rod if there is one at the main service panel. It it ok to ground the new meter/panel to the existing house ground rod and leave the ground rod for the subpanel connected? What is the correct way for grounding? Thanks in advance!