You don't need another battery, but you do need an uprated alternator. If you have just the fwd facing lights on(4 x roof 55w + 3 x bullbar 130 w + standard tail lamps + 2 x 65w standard headlamps) + the current needed to run the engine (EFI/Ignition), you will need about 90 amps continuous out of your alternator, we only have a 70 amp alternator at peak revs.... Idle the car (< 2500 rpm) for anymore than 30-40 minutes a night and you wont be able to start it up again. Idle it for more than 1 hour and it will stall due to flat battery. BTW, these figures assume that you ARE staring with fully charges batteries....

Having two batteries will lengthen the time you have until your batteries go flat, but they will still go flat. It is a supply/demand issue and you are trying to tackle the wrong end., The battery is simply a reservoir to capture excess alternator charge, and in your case you don't have any.....