If you are using two 6tl’s, place one in an inner spot and one in an outer spot. From outboard to inboard the terminals should go neg-pos-neg-pos The natural cable routing should allow easy connection to the batteries When facing the battery tray, nearest terminal of nearest battery needs to be a neg(-). That is where ground cable is connected. The next wire inboard toward the chassis is the 12V line. It will connect the plus(+) terminal of the outer battery to the negative(-) terminal of the inner battery.
The 24V wires(there are 2, one for starter, one to polarity box) closest to the chassis will connect to the plus(+) terminal of the inner battery. With 2 batteries, you will have 4 cable end connections not used, 1 24V, 2 12V, and 1 ground. These should be insulated to prevent an accidental short circuit.
The 24V nato slave/charge plug will have one wire connect to ground/outboard battery neg(-) terminal and the other cable connected to the +24V inboard battery positive(+) terminal, of course connected to give the proper polarity on the nato connector terminals...