Considering I have a yard full of heavy equipment only using 1 battery and an equal number of full size semi's with only 2 batteries, I think running 2 is more of a cosmetic thing or cover-up for other poor electrical system choices. Running a trunk mounted battery with 4ga will quickly make you think you need 2 batteries, so will running very poor grounds.
I was thinking of doing this to power the A/V and sound systems so that the kids could play games or watch movies while at shows and I wouldn't run down the primary battery. Good/bad idea?