My real-world experience over 20 years, always using multiple batts, is two 6's are better than two 12's. For reasons I stated, mostly because that you do not need to magically pair the 6's as you do parallel 12's.
Resistance and inner workings? What? Really? No. Im not running a carpentry shop here, just lights, fans, fridges and normal RV stuff that runs fine off of any 12 volt supply I have ever had.
Science and theory aside, two 6's have been far superior for me and my use than paired 12's.
IF you want science I think Trojan battery has a lot of info on their site, and any high-end battery site will have similar details, but you come to a forum to get real-world non-commercialized feedback. Sort through all the post I guess.