What you are doing is commonly done around here, with the 400-amp (320-amp continuous rating) meter socket fitted with double barrel lugs which feed (2) 200-amp panels.
It is usually sold as 400-amp service, but the GEC to the water main is actually sized like a 200-amp service(#4 copper) based on the size of the SE cable.
I connect it to one of the panels and run a #4 tap to the other panel.
The portion that goes to the ground rod is done the same way but need not be larger than #6 copper
