Take a look at this for using two batteries.
It allows you to use the two batteries simultaneously or with one main battery and one as backup.
Tech-aero's stuff has been rock solid for us. Since his battery regs worked so well we started using his ignition battery eliminator. Now we have over 20 planes with no ignition battery. Tried the Wike Mike stuff first and had multiple failures (Tech-aero was not making them at that time).