its just one big battery when you put em in series.. and if you have a single 12v battery and it fails your outta luck tooo so
@hemifoot's argument against 6v is nonsense.
I disagree; it's not nonsense - in his situation. Context.
And you said it right here in this quote; "if you have a single 12v battery"...
He doesn't.
So if your going to run two batteries anyway (which you must do to get 12v out of GC), then having 2x12v instead of 2x6v does provide a valuable redundancy. As Verkstad just pointed out, a single dodgy cell will put you out of business when you only have 6, but if you have 12 you can just halve your capacity and soldier on.
<general untargetted rant>
Running strings of 6v FLA deep cycle batteries is certainly an effective deep cycle setup (though strings of 2v is even better). But it is rarely a practical solution for most of the "overland" crowd.
Powering a travel trailer is one thing, but recommending a pair of GC batteries to a guy looking to power a fridge in a frikkin Samurai would just be unforgivably stupid...and would be a clear indication of someone who is more focused on talking than on listening.
I'm a notorious motor-mouth and make no apologies for that (marketing research shows that people have to hear something a dozen times before it sinks in)...but I do try to address the question that was actually asked.
</rant>