Cable size for series connection between 2 6 volt 400ah batteries

[mtt04]

I have 2 - 6 volt 400ah L16 AGM batteries. I have 4/0 cable for my connection to my Reliable Energy 3000 watt inverter.

What size of 10" - 12" cable should I use to make a series connection between the two 6 volts to make a 12 volt battery?

Don't worry about amp draw. 4/0 will handle the loads.

I was thinking 2/0