I admit I aint never messed with the 6.0 so I cant say anything about it for sure but every diesel I ever saw with two batteries had them both wired together in parallel so both batteries supply 12v power to everything with double the amp capacity for starting, except the series wired 24 volt systems like the John Deere 4010 or 4020 we have. So I doubt one battery runs the electrical BS and the other runs motor functions. If that is the way the 6.0 works then it is definitely different than the 7.3.
As for an extra and accurate volt gauge I like the one in my Prodigy P3 brake controller.
I would like to know if anyone has figured out how to make the factory oil pressure gauge work, that would be a nice mod.