Batteries: Two-6V In Series, or Two-12V In Parallel ???

The more I dig into this, the less worried I am in my decision to go with a pair of 6V AGM batteries. Phil, I admit I am a bit technically lost here, so I think maybe I will just assume it's going to be okay.

I still need to check my rig's on-board charger/inverter to see if it has an AGM setting. Though we very rarely use it for charging the batteries, it would be good to have it set properly if it can be made so.