2021 T@b 320 S battery questions: I'm going from 1-12v to 2-6v AGM. What do I need?

4 awg is good for 60 amps conservatively so that's more than enough. I think factory wiring is smaller 6awg. You don't need to change anything else unless you plan on adding a sizable inverter (500W+) which requires larger wiring.  

The factory wiring to/from the battery on my 2021 320S is actually 10 AWG.  I used 8 when I installed 2 6V batteries to tie the 2 batteries together to be on the safe side.  10 AWG or larger (8,6, etc) would be fine. 
The load in the T@B remains the same and therefore no change of the 2 inline fuses in the wires in the tub are needed.

Take it from them: