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

I don't know the output voltage of my various charging sources like the chassis alternator and on-board charger/inverter, but they are your typical sources, nothing unusual. The fellow who warned me stated his sources (same as mine) were outputting 13.8 volts while charging. I would assume that is close enough to 14 volts.

I suppose I shouldn't worry about it, just go about my business and see how these batteries perform.