The main panel already has 2 grd rods I'm running a 4 wire ser cable

The answer is maybe.

It depends on where the second panelboard will be.

If it is within the same structure as the main service panel, then no a separate grounding system is not needed. You still have to keep Neutral and Ground isolated from each other on the second panel but you do not need a new ground rod.

However, if you are installing the panelboard in a separate structure (garage, workshop) then the answer is Yes.

Ground and Neutral must also be isolated from each other.