I have never run mine on two batteries! I've run it in a rental car, as I posted before! I think you will find that a vast majority of those who spent a thousand plus dollars on a fridge also have dual batteries, solar, stc.
I figured a dual battery set-up is usually on the list of mods that come with a fridge. By the time you wire aux lights, winch, ect. the dual battery comes along somewhere on that list...