Another problem with this type of solution is that you must keep all parts of each system across the same battery or you will have common mode range problems ( the "ground" for the parts on the top battery is the "vcc" for those on the bottom battery.) Further unless you keep the applied loads on each battery very close to the same they will not discharge or charge at the same rate leading to premature failure, usually of the battery with the lighter load It is much simpler to use a 24/12 converter redily availble through large equiptment suppliers. 24 volt trucks ect all use 12 volt radios,lights ect. Terry